Feature request:

Currently variable kill-do-not-save-duplicates checks only the (car 
kill-ring) as docu explains:

Do not add a new string to ‘kill-ring’ if it duplicates the last one.
The comparison is done using ‘equal-including-properties’.

AFAIU it would be trivial replace this check by a call of "member", thus 
checking the whole kill-ring.

Thanks,

Andreas