I don't understand the use case for this - can you explain more with examples of where it would be useful?
The Find Duplicates plugin is about finding duplicate books. So I don't see series comes into this - you may or may not have the series name column populated so it should be ignored when looking for duplicate books. What has been asked for in the past is the ability to take series name into account when excluding books from being duplicates, but that is a very different thing to intentionally comparing series?
If you want to find duplicated series names (for instance so the automated numbering doesn't get screwed up when you add a new book manually, or if you publish to a decide using series as your primary identifier) then the Quality Check plugin has this functionality already.
So in what circumstances would identifying books by series name only be useful? And what defines "Similar" for a series name? It is not as if like an author you can chop out initials, reverse FN LN etc?
Just not quite sure what you are hoping to achieve with such an option?