"It's the real me, I assure you. I will soon celebrate my 76th birthday and I will still go strong," Buhari told Nigerians in a town hall session in Poland on Sunday, where he was attending a conference, when asked about the lookalike named Jubril.
Buhari, who is running for re-election in February, has been beset by ill health since taking office in 2015.
He was on "medical leave" in the UK for a few months in 2017. However, he did not reveal what he was suffering from, the BBC reported.
According to one theory that was widely circulated on social media, he was replaced by his lookalike. However, no evidence was presented.
"A lot of people hoped that I died during my ill health. Some even reached out to the Vice President to consider them to be his deputy because they assumed I was dead. That embarrassed him a lot and of course, he visited me when I was in London convalescing,'' he said, adding that those who spread the rumour were "ignorant and irreligious".
Former Vice-President Atiku Abubakar, 72, has emerged as Buhari's main challenger in next year's election.
The opposition is expected to make the President's health a major campaign issue, but he insists he is healthy and fit to serve a second term.
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