Posting Key for Sales Invoice

Hi,

I have a problm in creation of Sales Invoice

While creating Sales Invoice instead of Customer A/c getting debit & Sales Revenue getting credit the Sales Revenue Account is getting Debited & also getting credited. In accounting document the posting key are 40 Sales Revenue & 50 Sales Revenue instead of 01 for Customer & 50 for Sales Revenue.

Can I know where exactly I am wrong & where are the settingsd for posting key for Sales Invoice.

Thanks in advance.

Regards

Bala