Duplication in Acrobat is not like duplication in the Finder or Explorer. In the latter two cases, both OSs wish to make sure that the new copy/duplicate will not override the previous option/file. A field is neither an option nor a file.
Having no idea what the subject of your PDF file will be used for, let me make something up using "first name." If the internal structure of the form is all "First name" and that is copied. Then, your first name would be placed as your mother's first name, your father's first name, your sibling's first name, etc, because it will make a copy of that name field. It's just going to take that content, and any time there is a first name field, it will show what's been placed there in any place in the form. This is good because it eliminates having to reenter the same data over and over.
If, when you create the form, you provide content in the new fields, you can do a whole lot more. So, if you have a field for your mother and a field for your father, etc., in the end, you can have a list of all of the family's names simply by listing the fields. Nothing else has to be done.
And, if it makes you feel any better, I was banging my head against the table trying to get past this when creating my first form until I realized what was happening and why.
So, to answer the question and save time, you have to take a bit of time—just enough to make each distinct field unique. And one extra tip. "First name 1," "First name 2 " is NOT as good as "Member's first name," "Mother's first name," Father's first name," etc.
I hope that helps.
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