Partial Return procucts gives a warring

[Mayur Maheshwari(OpenERP)]

On runbut: trunk [build- 9029]

step to reproduce ---<>|

1] create a on sale order with product [[PC1] Basic PC , Quantity 50.00 ]
2] confirm sale order and delivered that product [[PC1] Basic PC , Quantity 50.00 ]
3] then return 10 product with 10 qty [no invoicing ] so it return it
4] so now product remains [40 qty ] so again return 10 qty so it's give a warring

       Constraint Error "Reference must be unique per Company! "

Thanks.
mma