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 has unveiled his crime policy, while Jeremy Hunt said cuts on policing had gone too far.
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 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.
'I don't vote twice'
Sir Patrick McLoughlin, who is chairing Mr Hunt's leadership campaign, admitted that he receives two ballot papers as he is on two separate registers.
"It doesn't mean I vote twice, I don't," he told the Today programme.
Asked whether the duplicate ballots need to be more heavily policed, he said: "It's right there on the ballot paper saying you must only vote on one occasion and I expect people to do that."
Mr Johnson's campaign chairman Iain Duncan Smith said he believed the Conservative Party chairman had "already been asked to look carefully at how they sift" ballots.
Speaking at a hustings in Cardiff on Saturday evening, Mr Hunt urged party members who have received more than one ballot paper to only vote once.
"I know that they won't vote twice, however tempting it might be to back Hunt twice, I'm asking them not to because we want this to be an absolutely fair election.
"Of course I'm going to trust the result," he added.
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 addressed the Young Conservatives Conference in Nottingham earlier on Saturday.
It came after Mr Johnson set out his plan to reduce crime in the Daily Mail, saying he would permanently restore stop and search powers nationwide.
Stop and search powers were restricted by Theresa May when she was home secretary in 2014. The powers have already been restored in seven areas with high knife crime on a trial basis.
Mr Johnson promised a "relentless focus" on knife crime and criticised the 2014 measures brought in by Mrs May.
Mr Johnson also plans to end the early release of violent offenders and address the causes of crime with a review of youth centre provision.
Speaking to the Today programme, Mr Duncan Smith - an ally of Mr Johnson - said police needed to be given "the capabilities to do their job", as well as there being an increase in police numbers.
When asked how Mr Johnson intended on funding 20,000 extra police officers, he said: "We've had to put the economy right from the terrible Labour crash that took place in 2007, but we are very much now back on track."
But Labour's shadow policing minister Louise Haigh dismissed Mr Johnson's policies as "meaningless" branding them "cheap headline-grabbing measures".
At the Nottingham hustings, Mr Johnson said the Tories should be presenting itself as a party that is "committed to social justice".
He said that would also mean "championing the environment", with measures to promote cleaner air, protect wildlife and reduce the amount of plastic being used.
"Our modern Conservative agenda is not only right for the economy, it's deeply progressive," he said.
Mr Hunt told the conference that government cuts had gone too far on social care and policing.
"I have been clear that we do have some headroom in our national finances that would allow us to find extra funding for those public services," he said.
He added that he "thought he could" match Mr Johnson's plan to recruit an extra 20,000 police officers.
Mr Johnson remains the frontrunner in the contest, with a recent YouGov poll of Tory members suggesting almost three-quarters of Conservatives back him.
But Sir Patrick said a "broad brush of people right across the whole party" have come out to support Mr Hunt.
He added that the foreign secretary was not wedded to leaving the EU by the "magical deadline" of 31 October, which he says is a "do or die" issue for frontrunner Mr Johnson.
He said Mr Hunt's "10 point plan very clearly" set out his plan for leaving the EU.
Sir Patrick said: "By the end of September he would decide, along with the Cabinet... whether we move forward with no deal or whether there was a chance of getting a deal".