My GF won't delete her ex's numbers?

My girlfriend and I are in a very serious relationship an have already been talking about marriage. We have both been through very bad relationships before, we both have been cheated on and both of us are slow to trust. The problem I have is that she keeps her ex's number in her phone and then texts them back when they text her! Its nothing bad she's always telling them that she is in love with me and not giving them hope of getting back together. Most of these guys just text to feel her out and see if we are still together. I have deleted all of my ex's that have ended badly, and i feel very paranoid about this because this is how it went with the last 3 gfs I have had, they all cheated on me and it started with me seeing a text from them. Am I being too paranoid? Is this normal?
My GF won't delete her ex's numbers?
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