Is it a red flag to be close friends with an ex?

If one person is close friends with their ex, I could see how it might seem like feelings are still there and cause insecurity in a relationship. At the same time, after ending a long term relationship, they might see them as more of a brother/sister instead of just never talking to them again and that doesn't seem so bad to me.

Is it a red flag to be close friends with an ex?
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