mmmm. It is possible for women to ovulate more than once in a cycle. It is not common but it does happen. I am wondering though are you certain that you got a positve result? In most tests the test line has to be darker than or equal in colour to the control band. I am sure you know this but I am just checking. I am using opk's at present too. In the past I have used them and never got a positive result even in the cycle that I conceived my now almost 3 year old daughter! I have been told by the manufacturer that it is often good to check twice a day the optimum time being 2pm as it takes time for the LH to be detectable in urine and then again at 8pm. I am doing this this cycle (from tomorrow).
If you are sure you ovulated then perhaps you have ovulated again. Maybe you have been given an extra chance this month! I hope so. I look forward to hearing how you go.
With love and blessings\
Happy1