I think there's a handful of blowers out there that sell their glass to many vendors on the site. In general, each blower does their own models, so if you see 50 vendors with the same glass piece, most likely it was made by the same vendor.
HOWEVER In the past such as the case with the original D020 - once it gained popularity, several knockoffs came about (and I still see several on Dhgate), they looked very similar to the original D020, but the build quality was way worse than the OG, clearly made by a different factory (funny, one chinese company copying the design from another vendor, way to steal your OWN designs for once!)
I haven't heard of any differences like that with the Double Barrel, so it's probably safe to assume it was made at the same factory.
Actually, come to think of it, here's a piece which I've seen at least 4 or 5 different versions:
They all have the same price, and look similar, though the vortex bulge near the mouthpiece changes shape - sometimes I see them smaller and more round, sometimes I see them long like this one. Also the recycler output is sometimes on the bottom of the rig, and sometimes raised up slightly like this one.
Here's a VERY similar piece, with a different perc - they could have been made at the same factory, but the pictures and build quality are different enough that one could have been a copy from the other