two billing document from same delivery.

Hi Experts,

Can any one suggest solution for below scenerio.

We have created one billig document againest delivery, also do release to accounting.

Again system is allowing to create billing documet for same delivery with same quantities.

We have checked all the possible rasons i.e. copy control from del. to billing & sap notes but did't get solution.

Item category -TAN

Billing quantity B

Pos./neg. quantity +

Pricing type C

Now Please suggest how to prevent system not to allow duplicate billing documents.

Thanks in advance.

Reagrds,

Dharminder dalal