Good idea Don!
It would be good to disable this button once the debugging session has begun and only re-enable it when the user presses the "stop debugging" button (Ctrl-Shift-F7) or Terminate button. This would prevent multiple presses of the debug button. This also incidentally solves the problem in the "Stopping debugging when run has stopped" thread and makes the suggestion there unnecessary (though you could do both ).
There are already safe guards in place which prevent multiple, concurrent runs outside the debugger.
Have a great weekend.
PS. Jeff, There seems to be two download links on the download page, one for 2.21 and one for 2.20 (at the top).
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David