Depends on the situation really. For most things outside of savage it really doesn't matter, though early on in heavensward a lot of people didn't want more than one brd/mch fro dps heavy fights like Bismarck...but that was mostly down to them having more of a stigma at expansion launch from minuet controversy and MCH being generally a bit underpowered. And pug unreliability :P

For savage itself - Bards and Machinists seem to be in very high demand right now, slightly followed by the melees, in my experience. As for doubling up, I don't think people would mind having 2 of the same melee, or even 2 blms for something like A1 - although most statics would try and avoid doubling up. It does slow down the limit break buildup. I'm sure it can and has been done though, especially now we've had a good couple of months of eso gear.