I agree and I'm sure it's an innocent oversight when people move and don't "unregister", at least most of the time. Eventually they should be dropped from the voter rolls in the original location.
The problem is some election boards don't review their rolls which makes them ripe for fraud. I believe that to be the case in places like Philly where virtually 100% of the adult population is supposedly registered to vote.
Also I'm not so sure the Florida snowbirds are your typical innocently double registered voters.
You can be sure they are not innocent. They are members of the AARP who usually vote solid democratic. Bush provided a prescription drug plan backed by the AARP to assist in breaking up this block. But the fact remains that double voting is illegal. Each poll should required a signed statement that the person did not vote in any other state. Then throw them in the clink when they make an "innocent" mistake and "forget" that they voted in NY. Of course they still have the advantage of voting in whatever state they believe needs their vote the most.
I travel constantly, but the law requires I have a single "state of residence". In my case it is Alabama. I will register and vote only in that state. Likewise, pay my taxes there. Snowbirds from a northern state and Florida should pick one state and make that their state of residence. Likely they will pick Florida for their favorable income tax status. Then they have to unregister in NY, even if that is where they happen to be in November.
In Philly, its not the percentage registered, its the percentage voting. What happens is that in certain precints there are literally no Republicans and the polls are not located in the most savory places. After the polls close, the RATS go inside and vote the entire roll. Thats why Though Philadelphia is a compact county it takes forevere for its results to come in. This is why the RATS complain so loudly at Republican efforts to monitor polling places. They claim it is racism, but the facts is it causes them to have to spend the time and money collecting the voters and bringing them to the polls.