An IDLE installation manual smoke test gives odd behavior when run on macOS using the Tcl/Tk 8.6.10 introduced with the new macOS universal2 installer variant as of 3.9.1rc1.  The test behaves as expected (without the duplicate and spurious error messages) when run using the traditional macOS 10.9 installer variant that includes Tcl/Tk 8.6.8 built on macOS 10.9.

The test consists of: 
- launch the new IDLE by double-clicking on its icon in the applicable /Applications/Python 3.x folder
- press cmd-N to open a new edit window
- enter a three-line snippet with a syntax error, something like:
  x = 1
  y = $2
  print(x,y)
- press cmd-S to save the file entering any valid file name
- press F5 to run the snippet
- observe the popup "Invalid syntax" error window
- click OK to dismiss and observe previous "Invalid syntax" window replaced by an identical error window [ERROR]
- click OK to dismiss
- remove the syntax error and press cmd-S to resave
- press F5 to run again
- observe the output of the print function in the IDLE shell window
- observe popup error window over IDLE shell window with "The Python Shell window is already executing a command: please wait until it is finished." [ERROR]
- click OK to dismiss and observe previous "already executing" window replaced by an identical error window [ERROR]
- click OK to dismiss
- press cmd-Q to quit IDLE

As we are planning to introduce the universal2 installer variant on 3.8.x and 3.10 to natively support Apple Silicon Macs and macOS 11 Big Sur, a quick resolution of this issue is important. While automated tests in the test suite show no new failures, it would be good to manually exercise IDLE in the new installer configuration to look for other unusual behaviors.

https://www.python.org/downloads/release/python-391rc1/