Index: src/wp-admin/js/theme.js
===================================================================
--- src/wp-admin/js/theme.js	(revision 28049)
+++ src/wp-admin/js/theme.js	(working copy)
@@ -801,7 +801,7 @@
 
 		// The left arrow key, previous theme
 		if ( event.keyCode === 37 ) {
-			this.previousTheme();
+			_.once( this.previousTheme() );
 		}
 	}
 });