Possible, the equipment and booking number is already present on another manifest. If your manifest is created via EDI, you are able to find this other manifest.
In the upper right corner go the ‘History’. To find out what happened you can search on: Equipment, booking number or the word ‘duplicate’ / ‘duplicaat‘. The word ‘duplicate’ indicates the equipment and booking number is not added to the manifest because it is already present on another manifest.
Via the information under ‘History’ you are able to find the cargadoor, Call Reference Number and terminal. With this information you are able to remove the equipment (yourself) from the other manifest.