How to make sure a user only votes once on a Poll system ?

Hi, All
  I have created a pool system that is only used by registered users and wondered what is the best way to make sure that users only vote once per poll ?.
 I would guess the best way would be by Ip ?.
Can someone give me some code examples or some information of the best way to do this please.
Many Thanks
Kered
 
 
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