I don’t live next to frum stores so managed to dress tzniyusdik by shopping non-Jewish.
There is hardly ever any overlap. Even if we buy the same brand, like CK or Tommy Hilfiger (what young people wear here) I will buy things that kids don’t wear: a longer cardigan etc.
That being said my high school daughter will sometimes wear my clothes or wear a similar outfit or we sometimes by the same item.
I feel very flattered! But we are not in the same school