Additonal line items generated when clearing GR/IR account via F.13

Question

Hi All,
We are running automatic clearing over the GR/IR account using F.13 and matching items based on the assignment number.  When a clearing document is created it is generating a couple of extra line items which appear to be identical apart from the sign. 
The additional line items are a debit and credit to the GR/IR account and when i compare them in BSEG they have identical data apart from teh posting key.
Is this how SAP clears open items in the new gl?  Since in previous versions of SAP i have worked with, the clearing documents have no line items.
Many thanks
Phil.

Answer 1

OK so the original documents are a goods receipt and invoice receipt which are a Credit and Debit to the GR/IR account as normal
The clearing document clears these off and posts two additional line items which are
PK   Account            Amount
40   20015 (GR/IR)   4298.89
50   20015                4298.89
So when i look at the clearing document and goto Environment > Payment Usage i can see the following documents
Assignment         DocumentNo          Type     Doc. Date      PK Amount in local cur. LCurr Tx Clrng doc.
          00000              1005722284      AB       26.03.2010   40            4,298.99  AUD        1005722284
          00000              1005722284      AB       26.03.2010   50            4,298.99- AUD        1005722284
850001577700020    5000706422      WE       16.02.2010   86            4,298.99  AUD        1005722284
850001577700020    5100804242      RE        19.02.2010   96            4,298.99- AUD   P1 1005722284
                                                                         0.00  AUD
The amounts in local currency, local currency 2 and document currency are the same in all line items.
thanks