Eder Lopes, 22, and Diogo Neves, 31, were both knocked off their feet after they connected with crushing blows to the head at the exact same moment.
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Cameras captured the pair rocking backwards before losing their footing during the bout in Portugal capital Lisbon over the weekend.
But it was double kickboxing world champion Lopes who emerged the winner after he got back to his feet quickly while Neves laid down out for the count.
Lopes was awarded the KO win while medical staff rushed on to help the stricken Neves.
The bonkers scenes occurred during the Portuguese brawlers’ fight under the ECE promotion – and fans couldn’t believe it.
One said: “This is like a scene from a cartoon.”
Another wrote: “This might be one of the best ever.”
Double knockdowns remain a rare event, though.
MMA fighters Matt Mitrione and Fedor Emelianenko sent each other down in their fight of the same year.
But Mitrione reacted faster by getting straight back to his feet and pouncing on his rival, pounding him with punches to secure the win.
And just a couple of months ago Steven Peterson and Martin Bravo performed near-synchronised spinning backfists during their featherweight clash at UFC Fight Night 159.
But Bravo was slightly off with his and Peterson landed his counter perfectly to record the KO.
Peterson later said of his unforgettable punch: “That was my best knockout. I’ve never had an out-cold knockout in my MMA career of 26 fights.”