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Britney Spears ’ ex Kevin Federline is rewinding to the night before their 2004 "I do" ... claiming he got one hell of a shock when he caught her on the phone with her ex, Justin Timberlake .

Chatting with " Entertainment Tonight " about his new memoir, "You Thought You Knew," Kevin says the late-night call raised serious red flags, and he told Britney he wasn’t down to get married if she was still hung up on her ex ... but she brushed it off, saying she was giving Justin a final send-off. Getty

Kevin says he thought it was pretty weird for Britney to be dialing up Justin the night before their wedding ... but he took her word for it, figuring she was just closing that chapter of her life.

Kevin admits he was head over heels back then -- but when it came to Britn

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