You can vote twice as long as they are for different elections. For example if there is a general election you can only vote once in it. But if it is held at the same time as local elections, you can vote legally in both the local elections.
I would think that the chances of being found out voting twice are low but not non-existent.
The cards are just an announcement that the election is taking place. They have no legal significance whatsoever - you don't need them to vote, and they don't prove who you are if you do use them, and the council are not obliged to send them out..