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And on that subject (accepted usage), how near are we to having to accept “me and John” did whatever? Shall we just give up on wanting to hear “John and I” did whatever? It’s nigh unto ubiquitous already, I’m sad to say. I mean, they just roared right past putting the other guy first (past John and I did whatever > past John and me did whatever >> right to me and John did whatever)… sigh
“Friend of John’s” is correct. It is not double possessive, because the “of” means “among” : “friend among many other friends.”
You will find a lot on this subject under these links:
“John and me” and “John and I” are good in that the first person goes last (our social norm). But “me” is an object, and “I” is a subject, so which one is correct depends on which is needed in a sentence. You will find discussions here:
Hope you have some fun with all that.