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I think this depends on the name, and obviously the person wouldn't just let me rename them if they already had a nickname they liked or a full name they only liked without the nickname, but I'd definitely try to avoid calling them by the same thing that the family member goes by.
My family members names are:
Theresa (Terri) - if I dated a Theresa I'd want to call her by the full name Theresa
Robert (Bob) - if I dated a Robert I'd want to call him Robert or Rob, maybe Robin. Any Robert who goes by Bob is too old for me anyway.
Jade (no nickname) - if I dated a Jade I'd want to call her Jadie

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I could also distinguish them by calling them "firstname-middlename" or "firstname-middle initial" but that's still basically a long nickname.