No question about people who might have voted TWICE ?

tRump told his minions to vote by mail and in person.

They are always checked. That's why provisional ballots are processed at the end. 

It is checked.  The safeguard is essentially the same for a person who tries to vote by mail and in-person as it does for a person who tries to vote twice in-person (either on two separate days at an early voting center or twice on election day at their precinct.  Every local election authority has a registry of voters and, when a person votes in an election, they are checked in on that registry (to make sure they can vote) and the fact that they have voted is recorded which prevents them from voting  a second time.  The exact rules for absentee voting depends on the state and when the ballot is received, but the general rules are as follows: 

In most states, if an absentee ballot is received in advance of election day (even if it is not formally canvassed before election day), it is noted on the voter roll that person X has returned an absentee ballot.  (Some states go as far as noting that a ballot was requested and require the voter to have the ballot with them to spoil if they want to vote in person.)  When such a voter shows up and tries to vote on election day, the workers in the precinct see that the person has already voted and does not allow them to cast a regular ballot.   (They can still cast a provisional ballot but will have to prove later that they did not cast an absentee ballot.)

In a minority of states, absentee ballots are not processed until after election day and, if the records show that the person voted on election day, the absentee ballot is rejected.  (In all states, the same holds true if the person voted early in-person and then tries to return an absentee ballot.) 

It is possible that some voters slipped through the cracks as there are always clerical errors, but it is highly unlikely that the numbers are significant enough to alter anything but the closest races (margins of less than 1,000 or 0.01% of the vote).  More people have their ballots discarded for technical reasons (ballot getting to the election authority one day late, not signing every place where a signature is required, failing to place the actual ballot in the security envelope) than are shown to have voted twice in post-election audits.   

Have you never voted in a U.S. election?  Do you not understand that it is almost impossible for someone to vote twice to vote for a dead person??  Unless all the people in a voting district are corrupt, I highly doubt that it's possible.

Everyone has the right to vote twice.

by Mail, and that vote should be held back until after the Polling.

If the person also voted in person, the in person vote counts, and the mail-in is destroyed.

However, in practice, if the mail-in vote is for the right candidate it is often tossed into the hopper to be counted also.   Once in the hopper it cannot be removed.

Like all forms of "widespread voter fraud", THERE IS NO EVIDENCE.