[runDialog] Don't add duplicates to the history

When the user runs the same command as the last one saved there
is no need to save it again, otherwise we the history might end
up having lots of dupes which makes searching for an old command
harder.

https://bugzilla.gnome.org/show_bug.cgi?id=613731

js/ui/runDialog.js

@@ -330,8 +330,10 @@ RunDialog.prototype = {
     _run : function(input, inTerminal) {
         let command = input;
 
+        if (this._history.length > 0 && this._history[this._history.length - 1] != input) {
             this._history.push(input);
             this._saveHistory();
+        }
 
         this._commandError = false;
         let f;