People should not be voting 15 times a day.
That doesn't happen, though. In-person voter fraud (what you are referring to) is by far the most inefficient way to steal an election, and it's harder to commit this kind of fraud than it seems. You actually need to be registered, you can't keep going back to the same polls, and post-election audits would reveal malfeasance if the state has no system to verify whether a registrant has voted or not, which itself would prevent double-voting.
This isn't the 60s. Dead people aren't voting by the tens of thousands and stealing elections. It may happen on a much smaller scale in some races, but again, this is not hard to catch after the fact
(or even beforehand, at that). There are ways to prevent this from ever happening without voter ID laws as well.
Trump is just whining about fraud because he knows he is going to lose. Republicans know there is little fraud going on, but they cry about it non-stop so they can justify voter suppression. The only people who are having an illegitimate effect on elections right now are Republicans with their voter suppression laws, which are disenfranchising far more people than those even trying to commit voter fraud.