Theoretically speaking, there are mechanisms that prevent someone from voting twice (you are only allocated to vote to one polling place based on your residence address, and that polling place has a name list where you sign off that you voted).
Are there any mechanisms in USA in place to prevent a person from voting twice by voting early by mail and then going and voting in person?
If the answer depends on the state, let's take NY, the home of SE the company.
Yes, there are mechanisms of varying effectiveness. For example, in California, when you apply for an absentee ballot, your name is removed from the list of voters at the polling place.
However, you may apply for an absentee ballot up to one week before the election -- which seems to be the standard for all states which I found information for, so far. By that time, the local lists seem to have been printed, because my uncle's name was still at the polling place (I checked) the first time he voted by mail. If he had wanted to, he could have also voted in person.
Now, this part of California uses a paper (Scantron) ballot with individual serial numbers. So, theoretically, an official could cross-reference the sign-in paper and see that the same voter ID voted both ways. This would be an intensive manual process, and I could find no evidence that it was ever done.
So, yes, there are mechanisms. And they do vary from State to State, and possibly county to county. For example, Oregon and Washington-state conduct all voting by mail.
Of course, in many states, there is little to prevent a person from registering twice with different names -- once for an absentee ballot and once for a normal ballot. You would only supply your driver's license or social-security number for one application. States like California allow no-ID registration(PDF doc), but you are supposed to then provide this ID at the polling place. (Except that ID is otherwise not required at the polling place.)
Officials could eventually discover that the same ID has voted twice. But, because there is an "air gap", manual correlation process, the odds of pulling off the fraud are good. I could find no record of my county's officials doing routine audits for this kind of double voting.
Yes, there are checks. My knowledge is based from the outside looking in (I'm a "deputy registrar" and I've worked as an election judge). At least here (in Dallas county Texas), any one who tries to vote is checked against the rolls. Early voting is entirely electronic and you can vote anywhere in the county. When you "check-in" you are checked against the rolls both for your registration and for your lack of a previous vote.
When voting"day of", the check is made against a per-precinct "book" of registered voters that includes their "has already voted" status. "Provisional ballots" (say, for a voter who lacks adequate ID) are packaged in two envelopes, one with full voter information and the second (inner) one that is completely anonymized. As I understand it, the process is once the ballot's "worthiness" is established (from the information on the outer envelope), the inner envelope is put in a ballot box for counting in a separate phase.
I believe that mail-in ballots follow a similar procedure; full information to check to make sure that the voter has not already voted, and then an anonymized ballot packet. With proper procedures, it's pretty easy to make sure no one double votes.
That said, mail-in ballots provide the vector for most voter fraud. Consider this article: https://www.dallasnews.com/news/2018-elections/2018/03/12/dallas-county-da-investigating-1200-mail-ballot-applications-potential-voter-fraud. As I understand it, mail-in fraud happens reasonably frequently (but in very low numbers). In person voter fraud pretty much never happens (though someone was convicted of it yesterday in Dallas (a convict on probation who voted even though she had no franchise)).
Pardon a somewhat sideways answer, but I think it's important to point out that the ballot system (like many pragmatic regulated systems) was never designed to prevent fraud in some absolute, draconian sense. The system was designed to make fraud unprofitable and ineffective. Most ballot-counting systems use a combination of partisan antagonism and overwrought bookkeeping to keep things in balance: with everyone watching each other with paranoid mindsets and a long paper trail to review and audit, no fraud on any meaningful scale can succeed. Individuals might get away with fraud on a random basis — if they don't mind risking fines and penalties if they get caught — but fraud en masse will raise red flags that will send the count into immediate review.
With respect to double-voting using absentee and in-person votes, both absentee and in-person votes are checked off on the district roster as they arrive. Those rosters are then rectified — compared to each other — while the counting process is ongoing, and anyone who has voted twice will see one or both of their ballots disqualified, and possibly be referred for prosecution. Catching this kind of fraud is really as easy (and mind-numbing) as comparing two lists (though it may be complicated in states that allow out-of-precinct voting). It's possible, even likely, that a few double-votes could slip through numbed-mind cracks, but even the most dulled mind is not going to miss large numbers of irregularities, and once irregularities in any quantity are noticed, the paranoid partisans running the show will wake up and start triple-checking everything. Sneaking a few bad votes through won't make a difference unless the final count is razor thin, and if the final count is razor thin, those same paranoid partisans will dig through the paper trail looking for ballots they can eject. It's just not a strategy prone to success.
