In most systems without a centralised authority, users are free to create as many accounts as they please, without any harmful effect on the system. However, in the case of e-voting, for instance, proof of identity is crucial, as sybil identities can be used to breach the intended role of the system. We explore the conditions under which a decentralised proof of identity system can exist. We also propose such a scheme, called Get-your-ID (GYID), and prove its security. Our system allows a user to generate and revoke keys, via an endorsement mechanism, and we prove that under some conditions which we discuss, no user can have more than one active key. We then show how voting protocols can be adapted on top of our system, thus ensuring that no user is able to cast a valid vote more than once.