A new wrinkle in developing a workflow for editing on a modern system and compiling within an emulator. If you tell ac to add a file to an image that already has a file with that name, it will add a new file with the same name rather than overwriting the existing file. For example:

$ ac -p test.po collar.jpg BIN < collar.jpg 
$ $ ac -ll test.po
test.po /TEST/
  COLLAR.JPG  Destroy Read Rename Write BIN  792 04/04/2021 08:26 04/04/2021 08:26 402,479 A=$2000 0002 0109 Tree  0 0
ProDOS format; 410,112 bytes free; 409,088 bytes used.
$ ac -p test.po collar.jpg BIN < collar.jpg 
$ ac -ll test.po 
test.po /TEST/
  COLLAR.JPG  Destroy Read Rename Write BIN  792 04/04/2021 08:26 04/04/2021 08:26 402,479 A=$2000 0002 0109 Tree  0 0
  COLLAR.JPG  Destroy Read Rename Write BIN  792 04/04/2021 08:26 04/04/2021 08:26 402,479 A=$2000 0002 0421 Tree  0 0
ProDOS format; 4,608 bytes free; 814,592 bytes used.

This also happens for -pt and -ptx.

I've not tested other image formats like DOS 3.3. So, this may be isolated to ProDOS.

Interestingly, nothing in ProDOS seems to mind multiple identically-named files... the shell I am using on the Apple II seems perfectly content to access one of the copies of the file. If I use a command to delete a file with the duplicate filename, it deletes one copy and leaves the other in place. Telling ac to delete the filename from the image does the same: it deletes one copy and leaves the other one there.

This isn't a huge issue, as I'll just have the script I've written to update files in the ProDOS image check for an existing copy of a file and delete it before adding an updated copy. But it might confuse people who are expecting that adding a file to an image will overwrite an existing file in the image, rather than creating a duplicate with the same name.