Check engine light

Hi, a few months ago a check engine light came on my car. Firestone ran diagnostic codes and replaced canister purge valve, vent shut valve and intake hose assembly. After about a month check engine light came on again, they ran the codes and replaced my fuel cap. Again the car was fine for a month or so and the light came on again. They ran the codes again and they said i have a faulty fuel tank pressure sensor and replaced that. That was last week and today the light came on again! Before I take it back to the shop I was hoping to figure out of they are just ripping me off by having me replace all these parts or is it really possible that all these parts are failing in such rapid succession? It seems incredible to me that all of these parts need replacing. Is it possible they are misdiagnosing the problem? Is it possible i just have a faulty check engine light sensor? thanks