Refer to the Inherit from an built-in Controller and SystemModules.Link - An object which has already been linked must be filtered out from the Lookup when it only can be linked once. issues for more details.
SystemModules.Link - Exclude objects that are already linked to an object from the Link ListView
Answers
Starting with version 12.1 you can control this behavior via the ExcludeLinkedObjects property of LinkUnlinkController. Take special note that this option has certain limitations, which are described in the B232305 ticket, so it is not enabled by default.