What kind of parts should be duplicated in case of a failure? God forbid anyhting goes wrong but I would like to be as pepared as possible. Thanx 
Anything extremely unique/critical to your robot, parts that are hard to make replacements for with rudimentary hand tools, and parts that are the most likely to fail in your design. Regardless of whether or not you have custom wheels or kit-wheels, it’s always a good idea to bring spares.
If you think it’ll break, you need a spare. If it’s impossible to break, have a spare. Last I checked, we had three of one part of our arm around (one practice, one real, one spare). This is an aluminum piece, square tubing, and small with one thing about it that’s difficult to replicate on the fly. I personally don’t think it’ll break, but we have a spare.
So would extra omni wheels be something that we would want to bring?
Definitely. We’re only using slightly-modified kit wheels, and I still hope we can get an order in for a couple more.
My guiding principle: If you can’t get it redone with parts from Lowe’s in five minutes, you should have a spare.