If you mean in the Relationships Diagram, then duplicating a field will simply create another field with the same attributes, with 'copy' at the end of the field name, ready for you to re-name it. No connections to any relationships are duplicated; neither is the data.
If you mean on the layout, then it will duplicate the field exactly, with its formatting, ready for you to change it to be a different field.
If you mean on the layout, then it will duplicate the field exactly,
And just to add some detail, if you duplicate a field object on a layout, it's a copy that refers to exactly the same field and record as the first and will thus show exactly the same data as the first. If you edit data in the first copy, the second copy shows the same data change in the same manner.
hmmmm....I can't seem to make it work.
I followed the instructions that were provided above (which are the same as the instructions provided by Apple under "Copying and pasting field schemas") and I still get the "unrelated table" error. see attached image.
There must be an additional step beyond the copy and paste functions for creating duplicate fields.
All I want to do is create a "title" field to go along with the other person-specific fields (phone number, mobile phone and email) in the contacts area.
Presumably you were in layout mode, you clicked on the 'Phone Number' field (for example), used Cmd+D or the Duplicate menu, and you were ether presented with the duplicated field and a dialogue box with a list of field names, or you double-clicked the new field to get a list of field names. The name of the original field will have been highlighted. Then what did you do?
<unrelated table> tells us that this field refers to a different table occurrence than that on which you layout was based. (and if you put a new field inside a portal, you have to pay attention to what table occurrence is the basis for that portal.)
Furthermore, it tells us that the specified table occurrence is not linked to your layout's table occurrence in a relationship over in Manage | Database | Relationships. Thus, it is reported as a field from an unrelated table.
Table occurrences are the "boxes" found in Manage | Database | Relationships. You generally start out with one for each table, but you can add more occurrences for the same table. Every layout specifies a table occurrence in Layout Setup | Show records from as it's basis or "starting point". A portal does the same in Show Related Records From in Portal Setup.
When you add a field to a layout, the specify field and field picker dialogs both first ask you to select a table occurrence by name from a drop down at the top of the dialog. Then you select a field from that drop down.
For more on table occurrences--a very crucial concept to understand if you want more from your database, see: Tutorial: What are Table Occurrences?