Duplication of Batch Number.

Hi ,

I have done GRN (101 Mvt type ) for a material on 05/05/2010 and the system has automatically generated a batch Number 000051820 and i have done the GRN for different on 10/05/2010 and the system has taken the same batch number 000051820, why is this happening , how the duplication of the batch numbers are getting generated , what may be the reason for this ?

regards

Rakesh