diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index b9b50dd485..5a9dc466ac 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -665,7 +665,9 @@ class WP_User_Query {
 				$cap_meta_query['compare'] = 'like';
 			}
 
-			$qv['meta_query'][] = $cap_meta_query;
+			if ( empty( $qv['meta_query'] ) || ! in_array( $cap_meta_query, $qv['meta_query'], true ) ) {
+				$qv['meta_query'][] = $cap_meta_query;
+			}
 		}
 
 		$meta_query = new WP_Meta_Query();
diff --git a/tests/phpunit/tests/user/query.php b/tests/phpunit/tests/user/query.php
index 51bcfeebe7..69c260a752 100644
--- a/tests/phpunit/tests/user/query.php
+++ b/tests/phpunit/tests/user/query.php
@@ -101,6 +101,9 @@ class Tests_User_Query extends WP_UnitTestCase {
 		$this->assertEquals( $names, $values );
 	}
 
+	/**
+	 * @ticket 21119
+	 */
 	function test_prepare_query() {
 		$query = new WP_User_Query();
 		$this->assertEmpty( $query->query_fields );