Why does Mercutio keep repeating, "A plague a' both your houses"?

Tybalt comes to challenge Romeo because Romeo went to the Capulets' party the night before. When Romeo arrives, he refuses to fight Tybalt because he is now related to Juliet's cousin by marriage, but because his wedding to Juliet is secret, he cannot say this.
Mercutio feels that Romeo, in refusing to fight, is behaving dishonorably and submissively, and so he steps in to fight Tybalt in Romeo's stead. Romeo comes between them both, trying to keep the peace and get them to stop fighting, and Tybalt mortally wounds Mercutio under Romeo's arm. Romeo effectively blocks Tybalt from Mercutio's view and makes it possible for Tybalt to strike him undetected. Therefore, when Mercutio says, "A plague o' both your houses," he is cursing both the Capulet family (Tybalt's family) and Montague family (Romeo's family) because he sees his death as a consequence of their feud. 
He seems to repeat the curse for emphasis. Perhaps he repeats it to make extra sure that it will take effect. Of course, we know later that both families will indeed be cursed when their children take their own lives because they cannot be together.