Some Conservative members have reportedly been sent more than one ballot paper for the leadership election.

More than 1,000 people could have multiple forms in the contest between Boris Johnson and Jeremy Hunt, according to a BBC report citing a party source.

The broadcaster said those affected could include people who joined their local Conservative party at their home and work address or others who have changed their name.

Hunt’s campaign chairman, the former cabinet minister Sir Patrick McLoughlin, warned party members should only vote once even if they had received two ballot papers. “It’s made very clear on the ballot paper that you are only allowed to vote once. I expect Conservative members to follow that,” he told BBC Radio 4’s Today programme.

McLoughlin acknowledged the error has “got to be looked at”, but added: “It can’t be clearer, it’s right there on the ballot paper saying that you must only vote on one occasion and I expect people to do that.”

Johnson is the favourite to win the vote, which closes on 22 July. The result will be revealed a day later.

Iain Duncan Smith, the former party leader and chairman of Johnson’s campaign, said the chairman of the Conservative party had been asked to look into the problem of multiple ballot papers being given to members.

“Obviously there is an issue here, I accept that,” Smith told the Today programme. “In all elections there are issues about this, where people are registered separately.”

Smith echoed McLoughlin’s call for Tory members to not vote more than once. “It’s quite clear as Patrick McLoughlin said that you are not to vote more than once.

“But it’s clear that the Conservative party needs to continue to do some work and look into how they can actually sift this properly so that they recognise who is who.”

The Electoral Commission, which sets standards for how UK elections are run, has no role in the leadership contest.

Theresa May will take her final session of prime minister’s questions on 24 July and hand over to the contest winner that afternoon.