A document was previously authored outside of Serna.
Duplicate ID was inserted when validation mode wasn't Strict.
If there is a duplicate ID in the opened document, validation mode switches from Strict to On, and a message reporting the duplicate ID appears.
Figure 1. Serna reports a duplicate ID.
You can click the message to call up the Attributes Dialog and correct the duplicate ID.
Figure 2. Correct the DITA ID in the Attributes dialog.
To enable the validation of DITA IDs within a map, you must first specify the ID value of the map itself.