EDIT

Reusing a taxon is not allowed in the same classification. If doing so anyway in the taxon node dialogue an error is thrown during save.

Better we should not allow selecting a taxon from the same classification from the beginning.
This can be done by either filtering the taxa in the taxon selection dialogue or by checking the classification immediately after returning from the selection dialogue by informing the user that the selection was not successful because the data is invalid.

As taxon reuse is rare the solution should simple (not take much time).