If you have been invited to a poll, you might notice that you are able to hover the mouse over some of the names and change the vote/name of that person. This is possible because Doodle does not authenticate the user, in order to keep the service as simple as possible.
When someone enters "Heather" as the user name, Doodle does not know whether it is Heather herself at the next visit. Therefore users can change the entries of other users. Doodle relies generally on cooperative behaviour of all users, which have been very successful so far.
As the admin, you can prevent participants possibly changing the votes of each other if you choose to make the poll a "hidden poll". See here how to make a poll hidden.
You can always protect your own vote when you participate in a poll while being logged into your Doodle account. Votes made from a Doodle account can only be edited by the owner of the account and the admin of the poll.