Import Proposals error Duplicate lookup reference

Hi All,

I would like to import a proposal (with general info) on different customers (CRM 2013).

First created a proposal on a customer with some general info. Made with advanced find an export with some general fields and the unique Account name and unique Accountnumber of the customer

I adjusted the xml file by adding (for trying) one extra row by copying the visible fields in an extra line and adjust this line with a known CustomerName and CustomerNumber. (I know that the invisible column are not copied and are empty)

When I wanted to import this data again with CRM it gave me the error message a duplicate lookup reference was found on the column CustomerName.

I hoped that the CustomerName and CustomerNumber was enough to get it unique and that he will create the proposal as well on the other account.

Is some able to explain to me what I'm doing wrong and maybe able to give me the solution how to solve this.

Thanks,

Patrick