A good and obvious way to stop people replaying missions and races to easily gain money and XP is to have the money and XP gained from it reduce with each replay. $1000 for the first playthrough, $900 on the replay, $500 on the 2nd replay, $200 on the 3rd replay and then $1 there after. XP should react similarly.
This is for back to back replaying. If you do a mission, then a different one or free roam before going back to the first mission, it resets back to the full payout.
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