Wouldn't it be interesting to have the possibility of having the same gear several times?

Basically many jobs, with different needs, share the equipment and therefore we often find ourselves having to re-melt the entire set.

Another case would be that the various contents (perhaps of the same tier) need different needs to be best addressed and re-melding a set for the various raids is not exactly the best.

Another also is perhaps based on the composition of the party.

In essence, having the possibility of having the bis always at hand (after healthy farming).

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Also wouldn't it be interesting if these equipment could be swap during fight?

In pve maybe there are phases (almost downtime) where having more hp/defense is better than having more dps.
This game actually has few mechanics that making could swapping profitable, in other games there are phases where the essential thing is to stay alive/make people stay alive and other phases where you have to push the dps to the maximum.

In pvp, swapping gear would allow you to face opponents based on the needs of the moment.