Everything depends on the system

  It really depends on how diligent were people who created that system. Technically speaking there is always a way to identify a voter to a certain extent and restrict extra votes. For example, even if the system doesn't require a code it can set a browser cookie to identify a voter and don't allow second vote, however if you go to browser settings and remove cookies you definitely will be able to vote again. Code can be a tricky thing too. It won't guarantee the uniqueness of the vote by itself. In other words code or no code, the system has a way to limit some extra votes or if they didn't bother to do it the system will allow everything.