Fix post tag duplication in ES query

includes/classes/Indexable/Post/Post.php

@@ -938,9 +938,12 @@ public function format_args( $args, $wp_query ) {
 		}
 
 		if ( isset( $args['tag'] ) && ! empty( $args['tag'] ) ) {
+			if ( ! is_array( $args['tag'] ) && false !== strpos( $args['tag'], ',' ) ) {
+				$args['tag'] = explode( ',', $args['tag'] );
+			}
 			$args['tax_query'][] = array(
 				'taxonomy' => 'post_tag',
-				'terms'    => array( $args['tag'] ),
+				'terms'    => (array) $args['tag'],
 				'field'    => 'slug',
 			);
 		}
@@ -1000,8 +1003,16 @@ function( $tax_query ) use ( $args ) {
 		$taxonomies = get_taxonomies( array(), 'objects' );
 
 		foreach ( $taxonomies as $tax_slug => $tax ) {
-			// Exclude the category taxonomy from this check if we are performing a Tax Query as category_name will be set by core
-			if ( $tax->query_var && ! empty( $args[ $tax->query_var ] ) && 'category' !== $tax->name ) {
+			// Prevents duplication of core's default taxonomies post_tag and category in ES query.
+			apply_filters(
+				'ep_post_tax_excluded_wp_query_root_check',
+				$excluded_tax_from_root_check = [
+					'category',
+					'post_tag',
+				]
+			);
+
+			if ( $tax->query_var && ! empty( $args[ $tax->query_var ] ) && ! in_array( $tax->name, $excluded_tax_from_root_check, true ) ) {
 				$args['tax_query'][] = array(
 					'taxonomy' => $tax_slug,
 					'terms'    => (array) $args[ $tax->query_var ],

tests/php/indexables/TestPost.php

@@ -4975,6 +4975,49 @@ public function testNestedTaxQuery() {
 		$this->assertEquals( 2, count( $query->posts ) );
 	}
 
+	/**
+	 * Test a tag query by slug using array and comma separated string as arguments.
+	 *
+	 * @group post
+	 */
+	public function testTagSlugQuery() {
+		$post_id_1 = Functions\create_and_sync_post(
+			array(
+				'post_content' => 'findme test 1',
+				'tags_input'   => array( 'slug1', 'slug2' ),
+			)
+		);
+		$post_id_2 = Functions\create_and_sync_post(
+			array(
+				'post_content' => 'findme test 2',
+				'tags_input'   => array( 'slug1', 'slug2', 'slug3', 'slug4' ),
+			)
+		);
+
+		ElasticPress\Elasticsearch::factory()->refresh_indices();
+
+		$query1_args = [
+			's'   => 'findme',
+			'tag' => 'slug1,slug2',
+		];
+
+		$query2_args = [
+			's'   => 'findme',
+			'tag' => [ 'slug1', 'slug2' ],
+		];
+
+		$query1 = new \WP_Query( $query1_args );
+		$query2 = new \WP_Query( $query2_args );
+
+		$this->assertTrue( isset( $query1->posts[0]->elasticsearch ) );
+		$this->assertTrue( isset( $query2->posts[0]->elasticsearch ) );
+
+		$this->assertEquals( 2, $query1->post_count );
+		$this->assertEquals( 2, $query1->found_posts );
+		$this->assertEquals( 2, $query2->post_count );
+		$this->assertEquals( 2, $query2->found_posts );
+	}
+
 	/**
 	 * Test a query with tag__and and tag_id params
 	 *