I don’t know anything about the accuracy of True the Vote’s charges here. But I do know that double voting across states is a real, though relatively small, problem.
Efforts to get states to cross check registrations is one way to solve the problem, but administrative errors and similar/same names and birthdays makes this not the ideal way to deal with the problem.
It’s why I’ve supported universal federal voter registration with a national voter identity card with each voter keeping a voter identification number for life.