I changed a number inside a contact on my mobile device and transferred only this contact again to my Gigaset handset, now the contact is doubled on my Gigaset handset, why?

As the APP and the base station will simply push the contact to the Gigaset handset the handset has to decide how to deal with this contact. If all numbers within the newly transferred contact are equal with the contacts numbers in the Gigaset handset, the newly transferred contact is ignored by the Gigaset handset. If at least one number in a contact entry is different to the number in the Gigaset handset contact, a new contact entry with the same name is generated. As it's not possible for the APP to delete single contacts, in this case it's recommended to manually delete the old contact entry (or entries) in the handset via the context menu in the directory before the contact is transferred again.