“This database is already linked using the Dedicated model” message is shown when linking a database to source control (4292277)

This message will be shown when the Link a database to source control command is used from the right-click context menu in the Object Explorer pane for a database that has already been linked to source control in a Dedicated development model:

To resolve this issue and link the database to source control again, the framework objects need to be deleted.

First, expand Databases and then navigate to Programmability | Database Triggers folder, select the ApexSQL_SourceControl_DDLTrigger object and delete it:

Then navigate to Tables, expand it, under System Tables select the dbo.ApexSQL_SourceControl_DatabaseLog table and delete it:

After deleting these two objects, the chosen database can be linked to source control again.