I don't understand what you are trying to achieve.
But if you copy a field on your lay out, you get the same field twice on your lay out, but it is still the same field from the database. If you want the the same field twice, but with different names and different values, you have to copy the field in your database structure. (Create a second field in your database).
just trying to save time in creating additional fields with a set of characteristics (size, format, etc.) without having to take the time of creating a new field, then setting all the characteristics. Seems like a simple inclusion in the program and I have found the effect in lots of other programs.
In the database structure you can also "copy" existing fields. These new fields in the database will have the same structure and characteristics as the one you copied. If you want extra fields with values you always have to create new fields, in every database.
On your lay out, you can copy an existing field , referring to a database field, and in the inspector you can change the database field it is referring to. Than you have te same structure, as well for the database field as for the layout field.
Making a copy of a given field on a Layout just gives you another instance of the same field. If you copy and paste in Manage Database you can create a new field then rename it. The new copy will have all the characteristics of the original. Why you would want to do this is another question . . .