Romeo and Juilet, more like Lancelot and Guinevere

When I was reading about how terribly sad Lancelot and Guinevere were in last weeks reading, I couldn't help but think of Shakespeare's "Romeo and Juliet". Lancelot was rejected by Guinevere, so he attempted to kill himself (if I'm remembering correctly). When Guinevere hears of this, she is so sad that she also wants to take her own life, BUT, what she doesn't know at the time is that Lancelot is still alive. How is this not the plot of "Romeo and Juliet"? He thinks she's dead, so he kills himself, but WAIT, she wasn't dead. I know there are some differences, but the similarities are impossible to ignore. This makes me ask the question I never wanted to ask myself; is Shakespeare not even original? Are all of his plays reminiscent of other works?