The message "Text is read-only" when repeatedly executing an irreplaceable buffer in python mode

Question

I am configuring emacs (24.3) in python mode. I would like to be able to edit the python buffer and execute it in the python shell using "Cc CC". It basically works. But I get the message "Text is read-only" if I stay in the python code buffer and execute the buffer again.

It does not matter if the buffer has been changed and saved, or if it is the same irreplaceable buffer, the Cc Cc repeats the error. The solution is like visiting the python output buffer and returning to the python code buffer and then executing - this works. But that breaks my flow. Does anyone else have the same problem? Is this some possible wrong configuration in my setup?

With respect, Roman