Those people must be guessing. I can't find any substantiation of that equivalence between fluoxetine and duloxetine. None of the studies available have included duloxetine.   As Cymbalta is an SNRI and Prozac is an SSRI, you run the risk of serotonin syndrome, as their effects on serotonin can be additive. This makes it tricky to switch from Cymbalta to Prozac.   Guides like this one advise tapering off the SNRI first, then adding the fluoxetine https://www.ncbi.nlm.nih.gov/