Yeah looks like Micheal Douglas was using that name professionally about decade before Michael Keaton started his career so union rules would require an alternate name.

Yep, that’s also why Michael J. Fox isn’t merely Michael Fox. I like to imagine that at some time in the 25th century or so, actors will end up having names that sound like generic pharmaceuticals because all the names will have been used already.

(As an aside, when Michael Douglas first attained prominence in the early 80s, I thought briefly that he was maybe the 70s daytime talk show host, Mike Douglas.)