with reference invoice when creating Creditmemo some line items duplicating

Hi ,

I have invoice with 4 items and with reference to the invoice I am creating credit memo request.

Credit memo request created with more than 4 items. This is happening some times only not every time.

e.g In invoice I have item 10. 20. 30. 40. When I create credit memo request, system created sales order 10. 20. 30. 40. 50. 60 (first 2 items copied twice)

Can anyone tell me the reason why it is coping few items twice and also not every time.

Best regards,

Alleiah