ya. but then with jumpp.in or jummp.in, it's hard to remember which of m or p is doubled

That's a better problem than having the whole domain name be hard to remember. I guarantee you a lot of people will look at this site and go, "Holy cow, that name is total nonsense," because the two doubled letters in a row make it no longer resemble the word "jump" so much as a cat walking on a keyboard. Personally, my skim-reading more closely associates it with the word "app" than "jump," which makes me want to read it as "jumapp."

My personal recommendation: For maximum readability, consider something like "letsjump.in". If you're attached to a quirkily spelled "jump in", "jumpp.in" is fairly readable.

i registered for both. so i now have

jummp.in letsjump.in jummpp.in

and incidentally, but completely unrelated,

hummpp.me ;)

which would should be the canonical? I am think letsjump.in? then in the future i can maybe buy jump.in, without having to rebrand

Nice. :)

Incidentally, I agree with all of the above commentary. Unfortunately, it's the sad reality of trying to start up an online service/product/whatnot today.

For the canonical, I like letsjump.in or gojump.in - and it's never too early to try and snag jump.in (the owner might be willing to part with it for less than you think). Good luck!