Telling him I just wanna be friends even though I sometimes flirt with him?

How do I tell a guy that's been very nice to me that I don't have any feelings for him? I know he likes me very much and asked me twice to be his girlfriend. It's my fault for flirting with him when my boyfriend and I went through tough time but we're okay now and I don't know how to tell the other guy I don't like him like that. I do wanna stay friends with him but I'm scared he might end up get mad and tell my boyfriend everything. Help?
Telling him I just wanna be friends even though I sometimes flirt with him?
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