Restore code that returns the existing media object if it already exists.

props programmin, adamsilverstein. fixes #24062

git-svn-id: https://develop.svn.wordpress.org/trunk@24447 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/js/media-editor.js

@@ -539,6 +539,9 @@
 		add: function( id, options ) {
 			var workflow = this.get( id );
 
+			if ( workflow ) // only add once: if exists return existing
+				return workflow;
+
 			workflow = workflows[ id ] = wp.media( _.defaults( options || {}, {
 				frame:    'post',
 				state:    'insert',