Yeah it bugs me too as I like to promote individualism. My mum always dressed me and my younger sister (by 14 months) in the same clothes but different versions. Like we had the same dress but hers was blue and mine was yellow, and she made some clothes and I had the dress whilst she had the romper style. I think that just pushed us close and into having to share everything. I'd rather have chosen my own clothes and my sister done the same, if we happened to like the same thing then great but I hated wearing the same stuff as her as I was never just me I was me and my sister. It was like we were twins when we weren't actually twins and I just wanted to be me sometimes. Obviously it stopped when we were old enough for my mum to let us pick out our own clothes. I think this is why I dressed the way I did as a teenager, it was because I wanted it to be as different from how my sister dressed as I could get. So even then it wasn't really my own style, it just wasn't hers....
...if that made any sense. I guess I'm still a little bitter lol. But yes, every time I see children in the same clothes especially when they aren't twins it bugs me x