G/L account determination for Vendor Billing document

Hello,

There are 2 requirements of the same nature.

1) there are duplication and unwanted G/L entries in accounting document

2) Where there is multiple entry, it is getting posted to single G/L account

How to rectify both. i had searched forums and help document, can someone help me with step by step procedure and this is from vendor billing document created from rebates.

Thanks in advance,

Subbu75