is this normal that each plugins has its own version ?
If the script isn’t registered by Core there isn’t much else a plugin/theme can do but register the script itself. If would be nice if there were some standardization, but that would be a tricky problem to solve.
what could be the issue for the original error : Uncaught TypeError:
jQuery(…).select2 is not a function ?
This is not likely to be directly WordPress related and I’d be guessing but see: https://stackoverflow.com/q/29968409/1735890