They're not collated.
To make life really fun for the people that work on these things, there is no overall database and each council area seems to think it's a fun idea to have a different format and different columns.
Local parties only get their own constituency database.
It is possible to do a crosscheck but would be like looking for a needle in a haystack unless you already had the name and addresses of the individual you suspected of voting twice.
The whole system needs a massive overhaul.
People do keep an eye on new registrations - friend told me a few weeks ago that they spotted 9 new postal votes registered to what they knew was a 1 bed flat. It was duly flagged.