How to stop overlapping?

How can you stop the overlapping of objects?
When I hold my mouse button it creates grass tiles works fine.
But the event keeps placing tiles even when there is already one placed.
How I can I fix this?

Example mfa (Look in the debug bar to see how many objects are placed):
http://download948.mediafire.com/y7r...mc/Example.mfa

I hope you can help me with this.
Thanks!

In short, create an invisible Detection Object which allways positions to the Mouse Cursor coordinates.
Then only create Tiles when the Detection Object is NOT overlapping other Tiles.

Thanks got it working.