The process for USA elections requires that the precinct election official has a list of people who are:
- Eligible to vote.
- Are registered to vote in this election.
- Live in the area covered by the polling location.
You are then matched to the list via your government ID, and have to sign something where they can compare your signature to that of the person-of-your-name who voted last time around. This serves as filling your 'slot', and thus nobody else can vote in that precinct under your name. If you go to the wrong polling location, they will be unable to match your name/ID to an eligible voter in that precinct. Using your ID, they can usually direct you to the proper location for you to vote.
If you are unable to vote in your proper precinct (for whatever reason), you can go to the county election board and vote using a provisional ballot. These are subjected to greater scrutiny, and are compared against the list submitted by the precinct at the end of voting such that someone who voted in both locations would have one of their votes thrown out, and submitted for voter fraud.
Now, is there a chance that someone who lived in Nevada and registered to vote there, then moved to a different state and registered to vote there, might have been missed by the Nevada voting commission and thus be left on the list of registered voters so they could vote absentee?
Yes, as we do not yet have a nationwide registry of voters, and while there are safeguards in place to prevent this scenario, county governments have varying degrees of competence in enforcing these safeguards. But as absentee ballots, they are treated to extra scrutiny, similar to the provisional ballot people, and they would be charged with voter fraud when they are caught. So while there is a non-zero chance of one or two people being able to vote twice, thousands of people 'falling through the cracks' is basically unheard-of.