It's not well supported as of today, but it has been discussed several times before in this list and in github tickets. Most recently:
https://groups.google.com/g/netbox-discuss/c/UyW3rjo_rac/I think that for now, the most pragmatic approach for duplex links is to connect the interface to just the odd-numbered port on the fibre infrastructure, and implicitly the even-numbered port is used as well. You could use a tag or a label on the even-numbered frontport to mark it as in use. (Unfortunately you can't mark a frontport as out-of-service like you can an interface; and you can't plug in a cable at one end to reserve it either).
The alternatives are too ugly to contemplate seriously. For example you could create two "interfaces" on the device, e.g. Gi1/0/1-Tx and Gi1/0/1-Rx. This is horrible because in reality there's only one interface and only one SFP.