Coincidental similarity with No Strings Attached?

I'm aware that most romantic comedies are similar to each other. I haven't seen No Strings Attached but from what I understand these two movies have an abnormal level of similarity even by romcom standards given that they are both about people who want sex without the complication of a relationship but find that staying emotionally uninvolved is harder than it sounds and then both movies end like any other romcom. Please note that I said I haven't seen NSA so if I'm wrong about the plot please set me straight. My points
1. I see that NSA was released 6 months before FWB. Does that mean that FWB had time to shamelessly copy NSA, perhaps iron out some of the faults which would explain its better critical reception and higher IMDb score. Or was it already in the works, written independently of the other one with the similarity coincidental? Or maybe the two movies both copied from some earlier movie about two people trying to have sex without a relationship.
2. Was the lower box office of FWB due to people having seen NSA and not wanting to see the same movie twice despite critics judging FWB to be the better movie?
3. I did not see either movie in cinemas and have only just watched FWB on DVD. I don't want to see the same movie twice and FWB looked like the better movie (btw I thoroughly enjoyed it. Perhaps its higher scores in general will see it rented more by people who haven't seen either. I notice it has more IMDb user votes.