Task showing up on different days.

I have set one task for 18 February 2013 (Monday), however it keeps showing up at 11 February 2013(Monday) and also 18 February 2013. No matter what I do, such as deleting it, re-entering it, nothing works. It is kind of irritating. I am sharing it with one other person. 

Edit: I noticed that it is happening for all the Mondays. What happens is that the tasks for 18 February is carried back and shown on both 11&18 February 2013. However this happens for every Monday in the calendar.