That is pretty vague. Are you trying to upload the exact same image?
The expected behaviour is: If you try to upload the exact same file, the upload is simply ignored and no error message shown. If you try to upload a different file with the same filename, you will get an error message.
Sorry, yes it was the same few files although I had resaved them so I’m not sure what Kirby does to detect it’s the same if it ignores the files modified time. Just tried again with a completely different file and I get the warning message. Sorry, I feel a bit stupid now.