EDI Order Import Error - 'You are trying to insert an existing order or update an order that does no

Hi,

I am trying to Import EDI orders into OM and I get the following message when the orders fail:

'You are trying to insert an existing order or update an order that does not exist.  Please enter a correct operation code. '

Has anyone experienced this error before as I cannot resolve it in the Corrections form. The operation code is INSERT which is as all other sucessful orders import as.

Many thanks,

EMG