[Koha] Deduplicate authorities

[Paul]

I'm not sure if this query is related to bug 7419, or just something that 
we are doing wrong (or not doing right.)  (Koha v. 3.6.1.)

Our cataloguers have started looking at "duplicate authorities" (mainly 
caused by import of Z39.50 biblios) and editing the biblios by using the 
drop-down "Tag Editor" at 100$a and "choosing" the one they wish to retain, 
then deleting the "old|unused" authority.

As an example, they started with:
Gibbs-Smith, C. H., 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles H., 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles H., 1909- 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles H., 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909- 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	4 biblio(s)

and ended with:
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	1 biblio(s) 	
Gibbs-Smith, Charles Harvard, 1909-1981 	Details 	4 biblio(s)

The incremental zebra (-z) crontab does nothing to deduplicate, nor does a 
complete zebra authority reindex (-a -r -v), and I have tried additionally 
(-b -r -v -x) -- no joy.

Is there a way to deduplicate?

Thanks and regards - Paul