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.