ERROR WHILE ADDING NEW NUMBERING SERIES

Dear All

I am working on SAP Business One 8.8 PL13 and am facing problem while adding numbering series, have added Period indicator as FY11-12 bt its giving error as 'This entry already exists in the following table NNM1'. If any of you know's how to resolve this error then please let me konw as soon as possible .

Thanks & Regards

DINESH CHANDRA