i pollwatched last weekend in ilinois. 49 ballots cast. among the 12 or so folks turned away (conflicting addresses, etc) were 3 folks who didn't know they had already voted. i got to hear this gem:
pollworker:"do you know you already voted?"
citizen: "um, i guess they already did."
49 ballots cast. 12 folks turned back, 3 had already voted. yeah, small sample size. yadayadayada...but voter fraud is real.