It is an offence for a person to vote twice in a general election. Any offence, if successfully prosecuted, could result in a fine of up to £5,000. At a general election the presiding officer at a polling station may, if necessary, ask any elector if they have already voted at the election, and may withhold a ballot paper from an elector if they do not answer the question satisfactorily.
If an elector votes in person in a polling station the electoral register is marked to record the issue of the ballot paper to them, and similarly a list is maintained of returned postal votes. In the event of an election petition or police investigation following a general election, these records may bring to light any voting irregularities at the election.