extra undo history added

1. Create [f] in a new patch.
2. Select it.
3. Choose "Duplicate" from the "Edit" menu.
4. Choose "Undo" from the "Edit" menu.

Bug: the 2nd [f] object gets repositioned over the 1st one.

Correct behavior: the 2nd [f] should have gotten removed after step 4 above.

Probably "Duplicate" is implemented by instantiating the copy and then using the "displace" widgetbehavior to move it down by (10, 10). Not exactly sure how to force the undo history to register an instantiation at the new coords rather than registering these two separate events.