Duplicate is a field which allows you to create a static duplicate of other field.
You will not be able to delete parent field of a duplicate untill the duplicate field is removed.
How to add duplicate field to the document¶
To add field to the document, use one of field adding methods with field icon in the Fields tab of template editor menu
Field creation form will appear, where you should set field attributes
Name - this is a name of a field
Document - a document where parent field is placed
Duplicate of - field which will be duplicated
If duplicate is placed inside of a dynamic table, only fields from the same table (or outside of all tables) will be available for duplication.
When all attributes are set, you can click Save button and field will be added. You can click field to see its properties and update them. Also you can delete the field in same menu.