You'd have to check a few things. If name display is used in your area, some providers will send the number twice if no name is available, once where the name would normally be displayed. Check to see if the number is sent twice in the Invite in 3CX Activity Log. It may also be added through a configuration in 3CX where you can normally add a DID name to the caller ID.
Does this happen on all sets, different makes/models, or only some? If only some, then it might be a set option.