Some Conservative members have been issued with more than one ballot paper to vote for the next party leader and prime minister, the BBC has learned.
One party insider estimated that more than a thousand voters could be affected.Members are warned that voting twice will mean they are expelled, the Conservatives said.
Meanwhile, Boris Johnson will unveil his crime policy while Jeremy Hunt will say it is time for serious leadership.
Ballot papers have been dispatched to around 160,000 Conservative Party members around the country to choose between Mr Johnson and Mr Hunt as the next leader - and the next prime minister.
The vote closes on 22 July, with the result announced the following day.
Some members have received two ballot papers, in some cases because members live and work in different constituencies and may have joined local Conservative Associations in both areas.
People who have changed their name, after marriage for example, may also have been affected.The BBC has also seen duplicate ballot papers which have been issued to the same person at the same address.