paper jam on duplex printing

Check the weight of the paper first. Is it standard copy paper or cardstock? Depending on the print engine (and even series if both models use the same or similar engines), some weights do not duplex well. For example, my Lexmark B3442 fails with decent cardstock (90 lbs or heavier) in the bypass tray. It can barely do it, which will pretty much be the demise of that unit for me when I run out of toner for them, and I'm ditching it entirely for an MS model (commercial grade like the MS621), which has a MUCH BETTER engine.
On the other hand, my C3326 can do 110lb cardstock very comfortably, but it will skew 110lb cardstock in the duplexer but not jam (but I also don't know if I got lucky when I had that issue); you need to step down to 65lb to do it with less fuss on the C3326. It's not as horrible as the B3442, for instance, but if I found the CS421 can do it (same engine as the C3426 and maybe the C3326), I'd switch in a heartbeat as soon as I have no leftover toner to worry about or go to the CS units which can do it.

Is it extra? Yes, but I want the option when the time is right, so if I need to run cardstock with some weight, I know my printer can do it. The reason is some prints I run (like my 3 letter agency drone certificate) are run on 65-90lb cardstock for durability reasons (and so I know it won't crumple with the drone) or 28lb laser paper with 7-10mil lamination; on the other hand, the ones I print to fit a wallet are run on 28lb laser paper with light lamination.

If the paper's weight isn't at fault, most duplexers are passive, not active (how you can tell if the duplex weight is notably weaker). If there is an obstruction in the path, this will occur. Check the duplexer path thoroughly. I'm unsure how Brother does it, but most disengage the duplexer from the bottom with a push tab. Check how your Brother does it, but it's probably under Tray 2.

If the issue remains even with common copy paper, it's probably weak pickup rollers. See if you can buy them with little trouble, but my experience with Brother is that, unlike HP, Canon, and Lexmark, you can't (readily) get some parts like the fuser. The rollers should be easier, though, given pickup rollers tend to wear well before things like the fuser, which usually lasts ~200k pages on most business-class lasers.