My ex boyfriend keeps flirting with my friends.. why?

Anonymous
So we broke up and didn't contact each other all summer then bumped into each other one night and starting seeing each other again. Our relationship wasn't the best and just before we made ourselves official he slept with a mate of mine.. and one of the reasons we broke up was because he was texting another friend of mine without me knowing and being flirty towards her whenever in company. yes.. I've realized he's a horrible guy and I was naive but he keeps flirting with different girls in my social group. why is he doing this? and how do I get him to stop?
My ex boyfriend keeps flirting with my friends.. why?
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