On October 18, U.S. District Court Judge Claude M. Hilton rejected a request from the Virginia Democratic Party for a halt to a purge of voters for which there is evidence that they are also registered in other states. See this story. The case is Democratic Party of Virginia v Virginia State Board of Elections, 1:13-1218, e.d.
Most of the voters were already “inactive” meaning that they had not voted in the last two elections and had not responded to a mail sent to their last address, or it was returned with an out-of-state forwarding address.
The few persons who contacted the local registrars were persons who had moved from Virginia, registered in another state, and then returned to Virginia. The act of registering in another state cancelled the Virginia registration, and the voter should have re-registered in Virginia.
One problem is that some states, including Florida, do not use SSN for voting registration records, so Virginia was unable to check those voters.
A national ID card would solve these problems. It is unlikely that persons would fail to update their address when they moved, and the federal government would automatically notify local election officials, so there would be no need to register or de-register.
Persons whose residence was ambiguous could simply remain registered at an original residence.