Now I've typed it, it seems petty.
DD has a classic name. We weren't sure how to shorten it at first, and she was known by both the obvious and the more old-fashioned nn at first. We stuck with the latter, thinking that if she wasn't comfortable with it, her friends will naturally call her by the more obvious when she's at school (she'll be enrolled with her full name).
Think Victoria - obvious nn Vicky, less obvious Tori; Isobelle - Belle or Izzy; Edward - Ned or Ed. You get the picture.
Anyway, my gran's old dog died just over a month ago and of course I was all tea and sympathy, only now she's replaced the dig with a pup, and gone and named her the most obvious, common derivative of DD's name.
I am genuinely asking myself if I'm being U to care about this. Perhaps what I'm more pissed off about is that she will have asked herself (and my auntie, who can notoriously convince herself that black is white) whether it might be a bit much. Well if I think it is, the answer is obviously bloody well YES! However, she's just gone ahead without even bothering to tell me anyway.
AIBU here? This is genuinely a name that DD may very well chose (have chosen??) to go by in the future. It is the one that people will most naturally and obviously shorten her name to unless she chooses to tell them that we use a different one. And WIBU to ask her to change it?
I know the likely response here - I don't have exclusivity on the name. Just seems a bit weird, s'all!
Oh and, ahem, I once jokingly called my pet ducks Joan and Ken after my gran and grandad (not this one, the other gran!) Hilarious I know 
, but maybe I've just answered my own question.