Even designers copy each other. It may be a trend and 'interpreted' their own way but there's always a leader and plenty of followers. That's why there're fashion trends.
I wouldn't buy dupes just because I am overflowing with bags 'till the next century. Every shape, size, colour and material. I'm good for generics. Whatever trend fashion dictates I'm sure I'll be able to source from my own 'archive'.
However, if there are a couple of bags I may have missed out on through the years, I'd happily do preloved. Hand-me-downs, inherited and buying vintage is where it started as a teen for me anyway. The quality of yesteryear cannot be compared to the highly available mainstream designer trends of today. I think you're talking about recent classics or trends so I won't bore you
I don't think contemporary bags are bad either, but some are untested for longevity if you're talking new brands/new models. Plenty of little bottegas in Italy and France, or independent leather makers in the US or UK all of who presumably follow trends too.
High-street - if you're a teen experimenting, have very little money/can't save or don't mind chucking after a season than maybe, but I have found buying the actual bag I really really want the best investment (new or preloved) and that's never been a dupe.
If someone wants a Chanel CF they are probably not going to be happy with a Zara copy, but if hey are just after the look, quilted pleather/leather chain strap with a ZZ where the CC should be, of course it's fine.