contacts – How to get rid of duplicates in my phone book?

After installing Skype a week ago, my phone book had duplicated many contacts. For most of them, I have seven duplicates, or eight entries for each contact.

After installing WhatsApp, I also encountered this problem: many contacts had one in duplicate.
But after installing Skype, the phonebook is a horrible disaster. For each of these contacts, Skype added five or six other duplicate contacts.

I tried to delete one of these duplicate contacts and I received the message "This contact is read-only. It can not be deleted, but you can hide it." – and below this message, the buttons "Hide" and "Delete" are displayed, but I have tried the "Delete" button and it works, so it can be deleted! Which is really inconsistent.

The question is, how can I get rid of all those duplicate contacts? (without having to delete them manually, one by one, of course)

On other phones, you can access WhatsApp from the main contact. There is no need to duplicate it to send a WhatsApp message from the main phone's contact list.

My phone is Honor 7s.

The Android version is 8.1.0

I use Nova Launcher, but the same goes for the Huawei Home launcher.