Fortunately, as in this case, there is always one or more solutions to solve the problem. The easiest and fastest way to remove duplicate contacts from the iPhone address book is to use a specific app. On the App Store there are several useful for the case. Among these I point out, for example, Cleanup Duplicate Contacts - Address Book Cleaner, Simpler - Contacts Manager and Smart Merge Pro. Using them is really very simple: the only precaution is to make a backup to avoid losing any important number by mistake.
The second method is to use iCloud: it takes a little longer, but it is also the safest method. Before proceeding, from the iPhone go to Settings, touch our name at the top and then iCloud. Make sure that the Contacts app is set to on. This assures us that the phonebook of contacts on the mobile is synchronized with that of iCloud.
Now with a pc we go to http://icloud.com. We log in with the same Apple ID as the iPhone and go to the Contacts app. We then check that the contacts are synchronized: the number of those present should be the same as those in the address book on the iPhone.
Before proceeding with the elimination of duplicate contacts, we make a security backup of the address book. Click on the icon with the gear wheel in the lower left corner, then click on Select all and then on Export vCard. This will allow us to export and save the entire address book on the PC in a vcf file that we will keep in case we accidentally delete some contacts.
Now we can proceed to clean up the address book of all duplicate contacts. We can select multiple contacts at a time by holding down the ctrl key. Once all the duplicate contacts to be deleted have been selected, press the Delete key and confirm with Delete.
If we have a Mac computer, the whole procedure becomes even much simpler. Let's open the Contacts application on the Mac. Then go to Card and select Search for duplicates. All duplicate contacts will then be searched and a popup window will ask us what we want to do. Just click Merge to merge duplicate contacts and put your address book in order.