At the polling place they have a list of registered voters and mark them off when they vote.

If people were voting under other people's names in significant numbers, it'd be obvious when the real voters came in and found out they were already marked. That very seldom happens.

The reason people don't do it is probably that an extra vote or two is unlikely to make any difference, so the risk/reward is poor. It's hard enough getting people to take the time to vote in the first place, much less risk a felony conviction to vote more than once.

It's not clear how requiring an ID would intrinsically stop duplicate votes?

IIRC a number of countries use an indelible ink pad that voters have to place a finger on.

there's little or no evidence that duplicate votes are a problem in US elections

You don't.

"BUT ITS NOT A HUGE PROBLEM"

it still happens, which is a problem none-the-less.

There's two problems. One is duplicate votes, which may be an insignificant problem. The second is public perception of the fairness and security of elections. The first is a problem; how big a one I don't know. The second is a problem, and a big one.