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.
BBC Radio 4’s Today Programme has learned that 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.
The Conservative Party and the independent body hired to scrutinise the running of the leadership election were both unable to say how many ballot papers had been sent in error.
“The ballot holds clear instructions that members voting more than once will be expelled,” the party said.
In most elections, voting more than once would be illegal, but the leadership contest is only governed by the Conservative Party’s internal rules.
The Electoral Commission, the independent body which oversees UK elections to ensure their integrity, has no role in the leadership contest.
As they seek to win support from the party’s members nationwide, both candidates will address the Young Conservatives Conference in Nottingham on Saturday.
Mr Johnson will set out a plan to reduce crime by permanently restoring stop and search powers nationwide. They were restricted by Theresa May as home secretary in 2014. These powers have already been restored in seven areas with high knife crime on a trial basis.
He also plans to end the early release of violent offenders and address the causes of crime with a review of youth centre provision.
Mr Hunt will tell the conference “we are in a time of real and present danger to our country” which calls for “competence, delivery and serious leadership”.
“It’s time to make a hard choice for the future not a feel-good one for the summer,” he will say.