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Finneas and Taylor Swift don't have any 'Bad Blood' between them ... but, he says his blood still runs cold whenever he thinks about an awkward encounter they had!

The 10-time Grammy winner sat down for an interview with The Guardian , published Saturday ... and, he reflected on a mortifying moment between him and T-Swift when he attended her birthday party. Getty

Finneas said Taylor thanked him for coming to her birthday party ... and, he responded, “Thank you for coming!” -- to her own party!

To be real ... we've all been there -- telling a server at a restaurant to enjoy their meal or an usher at a theater to also enjoy the show. It can get real darn awkward, but it seems Finneas is taking his blunder in stride.

BTW ... this was a joint interview with Finneas a

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