I think cynically we call it stealing, but as with anything, fashion repeats itself. Cycles occur, things ebb and flow, and you're likely to see that blast from the past from time to time. You see it with models, you see it with trends, so it's not surprising you would see it in the fashion designs themselves. While I find it humorous, I don't necessarily think certain designers steal for the sake of stealing, but rather, just having that moment of great minds thinking alike. However, certain designers do get pegged because there is a pattern that they've established over the course of their careers where it's extremely questionable whether they're ignorant of works from other designers and just happen to come up with the exact idea at a different period in time, or whether they're really copying.
I see your point on this, especially about certain designers who consistently reference other designers' work in their collections. It makes sense that history will repeat itself in fashion, for sure, but it still doesn't take away from the fact that the looks are very similar, even in terms of cut, fabrication, color, or print.