We have a 10.15.3 Mac with python 3.8.2, IDLE version 3.8.2, and we see similar behavior.

Summary:
The *second* 'bad' .py file you double-click won't open in IDLE. This seems unrelated to permissions, but a 'bad' file (saved by IDLE) can be converted to a 'good' file by emailing the file to myself.

Details:
Open IDLE, save a 5-byte file 'fails.py' (contents: pass) to an empty folder. Permissions are rw-r--r--.

Close IDLE. Double-click 'fails.py', which opens in the IDLE editor; a shell also opens. Double-click 'fails.py' again, or drag it onto the IDLE icon in the Dock, and it does not open in a second instance of the IDLE editor. Typing `idle3 fails.py` in the terminal does open in a second instance of the IDLE editor (same version as first instance).

Email 'fails.py' to myself, via Gmail. Download 'fails.py' from Gmail, and rename it to 'works.py'. Permissions are still rw-r--r--.

Close IDLE. Double-click 'works.py', which opens in the IDLE editor; a shell also opens. Double-click 'works.py' again, or drag it onto the IDLE icon in the Dock, and it *does* open in a second instance of the IDLE editor. Any number of instances can be simultaneously opened this way.

Close IDLE. Double-click 'works.py' several times to open it in several different instances of the IDLE editor. Double-click 'fails.py' once, which successfully opens in one more instance of the IDLE editor. Finally, double-click 'fails.py' a second time; it does *not* open a second instance of 'fails.py' in the IDLE editor.

IDLE's File->Open menu option does not open two instances of `fails.py` OR `works.py`; attempts to open the second instance simply raise focus of the first instance. However, if you open 'fails.py' via the menu, you can then open a second instance of 'fails.py' via double-clicking, but not a third.