Yahoo mail...5 times sent automatically by clicking few keyboard ons in a sigle time.how?

When i'm trying to send a mail to my friend yesterday, i mistakenly pressed few ons in the keyboards causing 5 times that mail had been sent to him.I also found 5 same sent items in my sent folder? I don't know which ons presses but the result is so strange.Do anybody experience like this before? How it is possible?