Just to update, it might have something to do with when you click the arrow to show more voters. Both of the polls mentioned above have enough voters that I needed to click the arrow to see everyone. Note that the first 25 avatars (those visible without clicking the arrow) have no duplicates within the first 25, and duplicates only appear after clicking the arrow.
Also, I just created an active poll that had two visible voters. The arrow appeared, indicating that a third person had just voted. When I clicked the arrow, five avatars total were displayed, including all three voters plus duplicates of the first two voters. Refreshing caused it to correctly display just the three voters, and since there was no longer an arrow there was no way to cause it to happen again.