Submitted by unforgiven, also found by csanuragjain, pcrypt0, and smilingheretic
Bribe.sol#L83-L90
Function deliverReward() in Bribe contract wont set tokenRewardsPerEpoch[token][epochStart] to 0 after transferring rewards. Gauge.getReward() calls Voter.distribute() which calls Gauge.deliverBribes() which calls Bribe.deliverReward(). so if Gauge.getReward() or Voter.distribute() get called multiple times in same epoch then deliverReward() will transfer Bribe tokens multiple times because it doesnt set tokenRewardsPerEpoch[token][epochStart] to 0 after transferring.
This is deliverReward() code in Bribe:
As you can see it doesnt set tokenRewardsPerEpoch[token][epochStart] value to 0, so if this function get called multiple times it will transfer epoch rewards multiple times (it will use other epochs rewards tokens).
function Gauge.deliverBribes() calls Bribe.deliverReward() and  Gauge.deliverBribes() is called by Voter.distribute() if the condition claimable[_gauge] > DURATION is True. This is those functions codes:
also Gauge.getReward() calls Voter.getReward().
condition claimable[_gauge] > DURATION in Voter.distribute() can be true multiple time in one epoch (deliverBribes() would be called multiple times) because claimable[_gauge] is based on index and index increase by notifyRewardAmount() in Voter anytime.
VIM
Set tokenRewardsPerEpoch[token][epochStart] to 0 in deliverReward.
pooltypes (Velodrome) confirmed and commented:
Alex the Entreprenerd (judge) decreased severity to Medium and commented:
