Please guide me guys, girlfriend has a male best friend that bothers me?

My girlfriend actually has a male best friend that i don’t like, he flirts with him as well and i know his intentions are not good, i’m about to propose my girlfriend but first i want her to remove any possible threats i feel that can harm our relationship, i trust her but it’s just that it bothers me. So how should i handle this situation, cos i know they both are kinda close friends
Please guide me guys, girlfriend has a male best friend that bothers me?
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