I was also registered in two states for a while. I had naively thought that registering in one state would automatically void registration in another state—until I got a letter from the one state that I was going to be removed from the voter rolls because I had not voted in five years.
As far as I know, I am registered in one state now.
I moved to Georgia from Penn, and continued to get correspondence from Penn about voting.
The States don’t seem to communicate with each other. Where no systems are in place, you can bet that fraud occurs.