diff --git a/src/wp-includes/class-wp-query.php b/src/wp-includes/class-wp-query.php
index 21127dc755f1c..482560f819db3 100644
--- a/src/wp-includes/class-wp-query.php
+++ b/src/wp-includes/class-wp-query.php
@@ -1112,8 +1112,7 @@ public function parse_query( $query = '' ) {
 
 		if ( ! empty( $qv['post_type'] ) ) {
 			if ( is_array( $qv['post_type'] ) ) {
-				$qv['post_type'] = array_map( 'sanitize_key', array_unique( $qv['post_type'] ) );
-				sort( $qv['post_type'] );
+				$qv['post_type'] = array_map( 'sanitize_key', $qv['post_type'] );
 			} else {
 				$qv['post_type'] = sanitize_key( $qv['post_type'] );
 			}
@@ -1121,8 +1120,7 @@ public function parse_query( $query = '' ) {
 
 		if ( ! empty( $qv['post_status'] ) ) {
 			if ( is_array( $qv['post_status'] ) ) {
-				$qv['post_status'] = array_map( 'sanitize_key', array_unique( $qv['post_status'] ) );
-				sort( $qv['post_status'] );
+				$qv['post_status'] = array_map( 'sanitize_key', $qv['post_status'] );
 			} else {
 				$qv['post_status'] = preg_replace( '|[^a-z0-9_,-]|', '', $qv['post_status'] );
 			}
@@ -1236,8 +1234,7 @@ public function parse_tax_query( &$q ) {
 
 			$cat_array = preg_split( '/[,\s]+/', urldecode( $q['cat'] ) );
 			$cat_array = array_map( 'intval', $cat_array );
-			sort( $cat_array );
-			$q['cat'] = implode( ',', $cat_array );
+			$q['cat']  = implode( ',', $cat_array );
 
 			foreach ( $cat_array as $cat ) {
 				if ( $cat > 0 ) {
@@ -1279,8 +1276,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['category__in'] ) ) {
 			$q['category__in'] = array_map( 'absint', array_unique( (array) $q['category__in'] ) );
-			sort( $q['category__in'] );
-			$tax_query[] = array(
+			$tax_query[]       = array(
 				'taxonomy'         => 'category',
 				'terms'            => $q['category__in'],
 				'field'            => 'term_id',
@@ -1290,8 +1286,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['category__not_in'] ) ) {
 			$q['category__not_in'] = array_map( 'absint', array_unique( (array) $q['category__not_in'] ) );
-			sort( $q['category__not_in'] );
-			$tax_query[] = array(
+			$tax_query[]           = array(
 				'taxonomy'         => 'category',
 				'terms'            => $q['category__not_in'],
 				'operator'         => 'NOT IN',
@@ -1301,8 +1296,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['category__and'] ) ) {
 			$q['category__and'] = array_map( 'absint', array_unique( (array) $q['category__and'] ) );
-			sort( $q['category__and'] );
-			$tax_query[] = array(
+			$tax_query[]        = array(
 				'taxonomy'         => 'category',
 				'terms'            => $q['category__and'],
 				'field'            => 'term_id',
@@ -1325,7 +1319,6 @@ public function parse_tax_query( &$q ) {
 				foreach ( (array) $tags as $tag ) {
 					$tag                 = sanitize_term_field( 'slug', $tag, 0, 'post_tag', 'db' );
 					$q['tag_slug__in'][] = $tag;
-					sort( $q['tag_slug__in'] );
 				}
 			} elseif ( preg_match( '/[+\r\n\t ]+/', $q['tag'] ) || ! empty( $q['cat'] ) ) {
 				$tags = preg_split( '/[+\r\n\t ]+/', $q['tag'] );
@@ -1336,7 +1329,6 @@ public function parse_tax_query( &$q ) {
 			} else {
 				$q['tag']            = sanitize_term_field( 'slug', $q['tag'], 0, 'post_tag', 'db' );
 				$q['tag_slug__in'][] = $q['tag'];
-				sort( $q['tag_slug__in'] );
 			}
 		}
 
@@ -1350,8 +1342,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['tag__in'] ) ) {
 			$q['tag__in'] = array_map( 'absint', array_unique( (array) $q['tag__in'] ) );
-			sort( $q['tag__in'] );
-			$tax_query[] = array(
+			$tax_query[]  = array(
 				'taxonomy' => 'post_tag',
 				'terms'    => $q['tag__in'],
 			);
@@ -1359,8 +1350,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['tag__not_in'] ) ) {
 			$q['tag__not_in'] = array_map( 'absint', array_unique( (array) $q['tag__not_in'] ) );
-			sort( $q['tag__not_in'] );
-			$tax_query[] = array(
+			$tax_query[]      = array(
 				'taxonomy' => 'post_tag',
 				'terms'    => $q['tag__not_in'],
 				'operator' => 'NOT IN',
@@ -1369,8 +1359,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['tag__and'] ) ) {
 			$q['tag__and'] = array_map( 'absint', array_unique( (array) $q['tag__and'] ) );
-			sort( $q['tag__and'] );
-			$tax_query[] = array(
+			$tax_query[]   = array(
 				'taxonomy' => 'post_tag',
 				'terms'    => $q['tag__and'],
 				'operator' => 'AND',
@@ -1379,8 +1368,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['tag_slug__in'] ) ) {
 			$q['tag_slug__in'] = array_map( 'sanitize_title_for_query', array_unique( (array) $q['tag_slug__in'] ) );
-			sort( $q['tag_slug__in'] );
-			$tax_query[] = array(
+			$tax_query[]       = array(
 				'taxonomy' => 'post_tag',
 				'terms'    => $q['tag_slug__in'],
 				'field'    => 'slug',
@@ -1389,8 +1377,7 @@ public function parse_tax_query( &$q ) {
 
 		if ( ! empty( $q['tag_slug__and'] ) ) {
 			$q['tag_slug__and'] = array_map( 'sanitize_title_for_query', array_unique( (array) $q['tag_slug__and'] ) );
-			sort( $q['tag_slug__and'] );
-			$tax_query[] = array(
+			$tax_query[]        = array(
 				'taxonomy' => 'post_tag',
 				'terms'    => $q['tag_slug__and'],
 				'field'    => 'slug',
@@ -2002,6 +1989,9 @@ public function get_posts() {
 			} else {
 				$q['post_type'] = '';
 			}
+		} elseif ( is_array( $q['post_type'] ) ) {
+			$q['post_type'] = array_unique( $q['post_type'] );
+			sort( $q['post_type'] );
 		}
 		$post_type = $q['post_type'];
 		if ( empty( $q['posts_per_page'] ) ) {
@@ -2293,6 +2283,37 @@ public function get_posts() {
 
 		// Taxonomies.
 		if ( ! $this->is_singular ) {
+			// Standardize prior to re-parsing the tax query.
+			$sortable_arrays = array(
+				'cat',
+				'category__in',
+				'category__not_in',
+				'category__and',
+				'tag_slug__in',
+				'tag__in',
+				'tag__not_in',
+				'tag__and',
+				'tag_slug__and',
+			);
+
+			foreach ( $sortable_arrays as $key ) {
+				if ( isset( $q[ $key ] ) && is_array( $q[ $key ] ) ) {
+					$q[ $key ] = array_unique( $q[ $key ] );
+					sort( $q[ $key ] );
+				}
+			}
+
+			$sortable_comma_separated_integers = array(
+				'cat',
+			);
+
+			foreach ( $sortable_comma_separated_integers as $key ) {
+				if ( isset( $q[ $key ] ) && is_string( $q[ $key ] ) ) {
+					$q[ $key ] = array_unique( array_map( 'intval', explode( ',', $q[ $key ] ) ) );
+					sort( $q[ $key ] );
+				}
+			}
+
 			$this->parse_tax_query( $q );
 
 			$clauses = $this->tax_query->get_sql( $wpdb->posts, 'ID' );
@@ -2640,6 +2661,10 @@ public function get_posts() {
 		if ( $skip_post_status ) {
 			$where .= $post_type_where;
 		} elseif ( ! empty( $q['post_status'] ) ) {
+			if ( is_array( $q['post_status'] ) ) {
+				$q['post_status'] = array_unique( $q['post_status'] );
+				sort( $q['post_status'] );
+			}
 
 			$where .= $post_type_where;
 
diff --git a/tests/phpunit/tests/query/parseQuery.php b/tests/phpunit/tests/query/parseQuery.php
index 94ced1ecd6e75..bbf3f1217fb2e 100644
--- a/tests/phpunit/tests/query/parseQuery.php
+++ b/tests/phpunit/tests/query/parseQuery.php
@@ -151,7 +151,7 @@ public function test_parse_query_cat_array_mixed() {
 			)
 		);
 
-		$this->assertSame( '-1,1', $q->query_vars['cat'] );
+		$this->assertSame( '1,-1', $q->query_vars['cat'] );
 	}
 
 	/**
diff --git a/tests/phpunit/tests/query/thePost.php b/tests/phpunit/tests/query/thePost.php
index 6032a01dbeb1b..84e824390b0a1 100644
--- a/tests/phpunit/tests/query/thePost.php
+++ b/tests/phpunit/tests/query/thePost.php
@@ -408,4 +408,120 @@ public function test_post_preview_links_autosaves() {
 		}
 		$this->assertSame( 'ticket 56992', get_the_content(), 'Permalink should show published content to logged out users' );
 	}
+
+	/**
+	 * Test that WP_Query::get() returns the value as passed on the `pre_get_posts` hook.
+	 *
+	 * @ticket 63255
+	 * @dataProvider data_pre_get_posts_includes_unmodified_query_vars
+	 *
+	 * @param string $query_var      The query variable.
+	 * @param mixed  $query_var_value The value to set for the query variable.
+	 */
+	public function test_pre_get_posts_includes_unmodified_query_vars( $query_var, $query_var_value ) {
+		$number_action_runs = 0;
+
+		/*
+		 * MockAction can not be used here because `$query` is an object and therefore
+		 * is passed by reference so will be modified by the time `MockAction::get_args()`
+		 * is called.
+		 */
+		add_action(
+			'pre_get_posts',
+			function ( $query ) use ( $query_var, $query_var_value, &$number_action_runs ) {
+				++$number_action_runs;
+				$this->assertSame( $query_var_value, $query->get( $query_var ), 'The pre_get_posts filter should return an unmodified query var.' );
+			}
+		);
+
+		new WP_Query(
+			array(
+				$query_var            => $query_var_value,
+				'ignore_sticky_posts' => true, // Ensures the sticky posts WP_Query does not run.
+			)
+		);
+
+		// Ensure the action was called.
+		$this->assertSame( 1, $number_action_runs, 'The pre_get_posts action is expected to be called exactly once' );
+	}
+
+	/**
+	 * Data provider for test_pre_get_posts_includes_unmodified_query_vars.
+	 *
+	 * @return array[] Data provider.
+	 */
+	public function data_pre_get_posts_includes_unmodified_query_vars() {
+		return array(
+			'post type, string'                 => array( 'post_type', 'post' ),
+			'post type, string[] DESC'          => array( 'post_type', array( 'post', 'page' ) ),
+			'post type, string[] ASC'           => array( 'post_type', array( 'page', 'post' ) ),
+			'post type, string[] duplicate'     => array( 'post_type', array( 'post', 'post' ) ),
+			'post status, string'               => array( 'post_status', 'publish' ),
+			'post status, string[] DESC'        => array( 'post_status', array( 'publish', 'draft' ) ),
+			'post status, string[] ASC'         => array( 'post_status', array( 'draft', 'publish' ) ),
+			'post status, string[] duplicate'   => array( 'post_status', array( 'draft', 'draft' ) ),
+
+			'post_name__in, string'             => array( 'post_name__in', 'elphaba' ),
+			'post_name__in, string[] DESC'      => array( 'post_name__in', array( 'the-wizard-of-oz', 'glinda', 'doctor-dillamond', 'elphaba' ) ),
+			'post_name__in, string[] ASC'       => array( 'post_name__in', array( 'elphaba', 'doctor-dillamond', 'glinda', 'the-wizard-of-oz' ) ),
+			'post_name__in, string[] duplicate' => array( 'post_name__in', array( 'elphaba', 'doctor-dillamond', 'elphaba', 'doctor-dillamond' ) ),
+
+			'cat, comma-separated string ASC'   => array( 'cat', '1,2' ),
+			'cat, comma-separated string DESC'  => array( 'cat', '2,1' ),
+
+			'category__in, int[] ASC'           => array( 'category__in', array( 1, 2 ) ),
+			'category__in, int[] DESC'          => array( 'category__in', array( 2, 1 ) ),
+
+			'category__not_in, int[] ASC'       => array( 'category__not_in', array( 1, 2 ) ),
+			'category__not_in, int[] DESC'      => array( 'category__not_in', array( 2, 1 ) ),
+
+			'category__and, int[] ASC'          => array( 'category__in', array( 1, 2 ) ),
+			'category__and, int[] DESC'         => array( 'category__in', array( 2, 1 ) ),
+
+			'post id, int'                      => array( 'p', 1 ),
+			'page_id, int'                      => array( 'page_id', 1 ),
+			'attachment_id, int'                => array( 'page_id', 1 ),
+			'offset, string'                    => array( 'offset', '5' ),
+			'offset, int'                       => array( 'offset', 5 ),
+
+			'post__in, string[] ASC'            => array( 'post__in', array( '1', '2' ) ),
+			'post__in, string[] DESC'           => array( 'post__in', array( '2', '1' ) ),
+			'post__in, int[] ASC'               => array( 'post__in', array( 1, 2 ) ),
+			'post__in, int[] DESC'              => array( 'post__in', array( 2, 1 ) ),
+			'post__in, int[] duplicate'         => array( 'post__in', array( 1, 1 ) ),
+
+			'post__not_in, string[] ASC'        => array( 'post__not_in', array( '1', '2' ) ),
+			'post__not_in, string[] DESC'       => array( 'post__not_in', array( '2', '1' ) ),
+			'post__not_in, int[] ASC'           => array( 'post__not_in', array( 1, 2 ) ),
+			'post__not_in, int[] DESC'          => array( 'post__not_in', array( 2, 1 ) ),
+			'post__not_in, int[] duplicate'     => array( 'post__not_in', array( 1, 1 ) ),
+
+			'author__in, string[] ASC'          => array( 'author__in', array( '1', '2' ) ),
+			'author__in, string[] DESC'         => array( 'author__in', array( '2', '1' ) ),
+			'author__in, int[] ASC'             => array( 'author__in', array( 1, 2 ) ),
+			'author__in, int[] DESC'            => array( 'author__in', array( 2, 1 ) ),
+			'author__in, int[] duplicate'       => array( 'author__in', array( 1, 1 ) ),
+
+			'author__not_in, string[] ASC'      => array( 'author__not_in', array( '1', '2' ) ),
+			'author__not_in, string[] DESC'     => array( 'author__not_in', array( '2', '1' ) ),
+			'author__not_in, int[] ASC'         => array( 'author__not_in', array( 1, 2 ) ),
+			'author__not_in, int[] DESC'        => array( 'author__not_in', array( 2, 1 ) ),
+			'author__not_in, int[] duplicate'   => array( 'author__not_in', array( 1, 1 ) ),
+
+			'tag_slug__in, string[] ASC'        => array( 'tag_slug__in', array( 'bobby', 'hans', 'herman', 'victor' ) ),
+			'tag_slug__in, string[] DESC'       => array( 'tag_slug__in', array( 'victor', 'herman', 'hans', 'bobby' ) ),
+
+			'tag__in, int[] ASC'                => array( 'tag__in', array( 1, 2 ) ),
+			'tag__in, int[] DESC'               => array( 'tag__in', array( 2, 1 ) ),
+
+			'tag__not_in, int[] ASC'            => array( 'tag__not_in', array( 1, 2 ) ),
+			'tag__not_in, int[] DESC'           => array( 'tag__not_in', array( 2, 1 ) ),
+
+			'tag__and, int[] ASC'               => array( 'tag__and', array( 1, 2 ) ),
+			'tag__and, int[] DESC'              => array( 'tag__and', array( 2, 1 ) ),
+
+			'tag_slug__and, string[] ASC'       => array( 'tag_slug__and', array( 'bobby', 'hans', 'herman', 'victor' ) ),
+			'tag_slug__and, string[] DESC'      => array( 'tag_slug__and', array( 'victor', 'herman', 'hans', 'bobby' ) ),
+		);
+	}
 }