Duloxetine is one of the newer ADs, has different uses at different levels. It is generally seen as a 2nd tier/level AD, if citalopram or fluoxetine not proven to help etc. It was originally producd for incontinence and then its AD qualities at different doses were discovered. It is akin to venlaflaxine which is an SNRI or mirtazapine.
However, am sure your gp will want to know the rheumatologists reasonings as it is a different AD etc.