Ha Nicole, what an interesting question! I have been thinking about something very similar these days anyway.
First I'd like to say that I don't mind at all and am actually flattered when a YLF online friend wants to but something I have posted a photo of
With people you actually know in reality, it's a bit different. I am sort of in the middle and I'd say it depends on the context and situation. I'll give two examples.
First, in college, one of my good friends was basically being my copycat. Almost anything I would buy, she would go out and buy the same or extremely similar thing. Or we would browse stores together, and I'd point out something I liked, and later she would go and buy it for herself. This was annoying me to death. I figured it was because of her own insecurities, but still. We had very small wardrobes in those days and could buy on average one item every second month, so we had very similar wardrobes. Quite a few times we would show up at places dressed almost the same ( without having coordinated it). Or I was saving to buy something and she would snatch the last item right in front of mu nose. And then she would tell friends that my style was 'out there' while hers was very simple, timeless and modern classic. Needless to say, I absolutely hated the situation.
My best friend here is doing a big wardrobe transition and I am her own 'stylist', we go to malls together and I'd help her pick out stuff, recommend things to try, express my opinions and so on. She values my opinion a lot but of course gives the final say which I like. So here and there we would get some same items, sometimes while shopping together, and sometimes separately, so I don't mind it a single bit - I am very flattered in fact, especially since she is 8 years my senior. We work at different places and see each other 1-2 times a month, so chances of wearing the same thing in the same situation are almost non-existent.
I have to admit that, in reverse, I have few female friends./coworkers, and most are pretty bad dressers so I rarely want to get something they have. On the other hand, I get attracted to quite a lot of things YLFers post here. But I would only get something if it actually works for me, not just because someone else has it/looks good on them and I don't have a better idea myself.