I cleaned up your function - it can be much simpler.

# make a new ( numbered ) copy of the original file
def uniquify( path, ext ):
    file_name = path + ext
    i = 0
    while isfile(file_name):
        i += 1
        file_name = path + "_copy_" + str(i/10) + ext
    return file_name

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)