Anyone whose mailed-in vote is already counted will be turned away on November 3rd. They won't be given a chance to vote a 2nd time. However it will create mass chaos at the polls on election day, maybe that would be Chump's real reason for telling people to do this.
By simply mailing in our votes and avoiding the polls on election day, we'd be diminishing the chaos. I plan to use my mail-in ballot and personally hand it to the County Election Board officials, that way I don't have to depend on the "good will" of the US Postal Service.