Maryland this year became the first state to join a multistate database that should flag voters registered in more than one state, Maryland voter registration director Mary Cramer Wagner said.
But the Electronic Registration Information Center, or ERIC, would not have found Rosen, because Florida has not joined the database.
Seven states have signed on; more are expected to join. Wagner said the Maryland State Board of Elections should receive its first report as early as this week.
A spokesman for Maryland Democrats said party leaders were told of Rosen's dual registration on Friday by someone within the party. After verifying the details over the weekend, spokesman Matthew Verghese said, the leaders confronted Rosen on Monday.