Remember, in order to stack Duelist, you need to trade. Considering that Duelist is a HoT, if you're going against a bursty champion (or even Singed, for goodness sake), your HPS isn't enough to give you that advantage over your opponent. Most of the time.
Here's an idea: why can't we let Riposte reflect ANYTHING? Sivir and Nocturne both have shields that will block an ability... let's add that feature to Fiora, considering she has absolutely no escape mechanism. Similar characters, such as Yi, Olaf, and Jax, all have escapes or mechanisms that can be used to escape... where's Fiora's? (Especially considering that Fiora's bases are even less tanky than other options... ouch.)