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Is it possible to set unique database field option retrospectively

You cannot set the "unique" (i.e. unique field contents) database field option retrospectively if there are already several documents that have the same field contents in this field.

When you set the "Unique" database field option, you decide whether the condition is checked by DocuWare or by the database. Even when the "Unique" option is set DocuWare will accept multiple documents without an index entry in the relevant field. The process of checking for pages in the database varies according to the database: 
An MS SQL database treats an empty field entry the same as a normal field entry. It is therefore not possible to store more than one document in a file cabinet without an index entry for a field assigned the "unique" option.
With Oracle and MySQL databases on the other hand, even multiple documents without an index entry in a field with the "Unique" option can be stored in the file cabinet.
This can therefore lead to problems with the following scenario if the check is run by the database and not by DocuWare:
A Microsoft SQL database is being used in a central DocuWare system. Single workstations/mobile user clients are using MySQL databases. The clients store documents in the satellite file cabinet without index data in a unique field. At synchronization, the central file cabinet will be unable to import these documents.