sale invoice pick serial number two time
issue is when i m genrate a sale invoice of particuler trator material t100001 with the serial number AM1005 dated 11-3-2008 .
and after 11-3-2008 again dated 12-3-2008 when i create new sale invoice of particuler trator material number t100001 then system again show me same AM1005 serial number which one i m already used in pervius sale invoice date 11-3-2008
how it can poosible if once a sale invoice genrate with paticuler serial number so why the again genrate sale invoice same serail number
why the sale invoice pick same serial number in two time ?
kindlly tell me how can i resolve this problem ?