I've run into this combination of words before but I've simply changed what I was writing to avoid this. There's clearly a better way to write this meaning. So what is it? "All there is, is love." "All there was, was love" Is is a matter of the comma?
You could rewrite it as "All that existed was love." If you're going with what you have, I wouldn't put the comma there. Treat it like you would "that that," which also has no comma.
"There was only love"? Or possibly, "Love was all there was." Though that one seems almost as clumsy as the original.
Replace "all there is" with "_____". "Love is _____." "_____ is love." Without the fact that it just feels awkward to write the same word twice in a row, there is no reason to use a comma. "All there is is love" is valid. However, my favorite way to say this is probably "There is only love."
Double words are always problematic in writing. He had had measles before. Of course you can use a contraction in this case. He'd had measles before. I'd say just work out a different way to say it, if you can. All that remains is love. Something like that.