This error on the E-Invoice import means that the provider has used the Invoice Reference previously. This could be either in a previously imported file, or a having been typed in as a reference on a manually entered invoice in Mosaic.
The invoice will have been rejected by the E-Invoice Import and will need to be entered in to Mosaic in one of two ways:
If it is the whole file that has been rejected, then the E-Invoice Import file could be updated to contain unique reference numbers and reloaded.
If there are only a few already used references, then best approach will be to manually enter these in to Mosaic front end with a unique reference number.