OK, I went back to the site by clicking on the link in my email, and got this:

Stem op jouw favoriete kat.
Oeps er is iets misgelopen!

Sorry. Je kan maar 1 stem uitbrengen.

Het Whiskas Team.
So - it has recorded my vote, and is not letting me vote twice?

"Oeps" and "Sorry" seem to indicate that!