To make sure the passengers from the Holcar integration are linked with the correct contacts/users you can add the "Holcar Passenger name" to the corresponding contacts.
When the Holcar integration has been setup, you'll find a new field for each contact in the contact module named 'Travel name'. When you add the travel name for a contact, for example 'POST/PIETER MR' (This name will be generated from Holcar, they can send you these names for each passenger) for your contact 'Pieter Post', the integration will find the correct contact to link as a passenger to the flight or hotel.
You can find the Travel name field in the contact modules, located on the right below the notes field.