Import Excel - Unique Resource Ids

I am importing Resource assignments into P6 v2.1 but P6 will not import the same resource id twice. This may sound as if it’s an in built function. However, within P6 you can add the same resource many times to one activity so why can’t I do this via import.

The error message is:
Excel Import log started : 25-Jun-10
INFO: Importing Global Data...
INFO: Importing Project Data...
INFO: Importing:
INFO: Saving:
ERROR: Failed to find a matching object for Activity Resource Assignment
ERROR: Failed to find a matching object for Activity Resource Assignment
INFO: Saving Data to Database...
INFO: Deleting Unreferenced Data...