Use of tabbed browsing results in form confusion

If using tabbed browsing to "multi-task" in CiviCRM admin area, the first form opened will act like the second form opened on submit.

Steps to duplicate:
1. login to sandbox site
2. go to admin -> custom data -> relationships extra
3. open "edit field" on a field in a new tab, but do not submit yet
4. open "create a new field" in a new tab
5. submit the tab opened in step 3

Actual behavior:
Duplicate custom field is created as if the new field form was submitted

Expected behavior:
Original custom field opened for editing in step 3 should be updated

Affects sandbox rev 19272 and 2.0.5 for sure, probably versions in between as well