> On Apr 12, 2021, at 5:32 AM, Lars Ingebrigtsen <larsi@gnus.org> wrote:
> 
> I was unable to reproduce this (in Emacs 28).  I pasted this into the
> *Python* buffer:

Pasting multiple lines constitutes a single command for post-command-hook. In that case you would not expect to see any repeated lines.

> Do you have a step-by-step recipe, starting from "emacs -Q", to
> reproduce this problem?

emacs -Q
M-x run-python
C-x 5 b “ *Python-font-lock*”
In inferior python shell: type any line.
C-c SPC (`comint-accumulate’) to continue.
Type another line.  Notice the first line is repeated. 
Repeat steps 5 & 6 several times. 

The issue is how input is gathered from the prompt to the end of shell buffer in the post-command-hook:

      (let* ((input (buffer-substring-no-properties
                     prompt-end (point-max)))

Line-at-a-time also won’t be able to handle editing lines before the last.  On emacs-devel Stefan has been helpful with some ideas to perform font lock in-place in the shell buffer, which is working well in my tests.  I’m using that as part of a package to implement true multi-line editing (ala iPython).