That is actually correct:
[BEGIN QUOTE]
“It’s not illegal. People often move out of state, register in their new state, and then just vote in their new state,” Allegra Chapman, director of voting and elections at Common Cause, said in an email. The old registration eventually gets purged when states “clean the rolls,” she said.
Gerry Hebert, director of voting rights and redistricting at the Campaign Legal Center, said duplicate registrations usually were clerical errors.
“It’s a matter of election administration (or lack thereof) when someone moves and their former voter registration office never gets informed by the new voting registrar,” Hebert said in an email.
[END QUOTE]
At one point, I was registered in PA and NM because NM did not get informed properly. I only found out I was still registered in NM because my parents told me they were asked why one registered voter at their address had not voted in something like 4 straight elections when everyone else had.