Sometimes while typing quickly in PM's the last line which you have typed will be retyped automatically. This most often happens when typing a long sentance, an enter, and then a new sentance.
As far as I can recal this bug has been around for around 8 years or more (possibly since the very beginning, not sure). In any case, unlikely it wil get fixed now if it hasen't been yet. But you never know ^^
