Internal audit vs external

Hello guys I need your input here. My company policies and procedures are based on NIST CSF and we have been auditing internally against it. Recently an external auditor is expecting us to do another internal audit against ISO 27001 despite the fact that we mapped our controls to reflect ISO. It just doesn’t make sense to me. I have been asking them to prove me where that rule is written, I’m still waiting to see it.
I think it …

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