Remove DAVdroid. Then try to wipe your contacts storage (Android settings / Apps / Contacts storage / Wipe data). This will remove all your contacts from the phone (not on the server of course), also other contacts not accociated with DAVdroid will be deleted. Restart your device and then make a new account with DAVdroid. Maybe this helps you getting rid of these ghost-entries.
Yess! This procedure effectively removed all ghost entries!
In fact in the past the phone was used by my girlfriend who used a different phone book, always davdroid.
Many thanks for your help!
I’m much quieter now that I know it does not depend on a Nextcloud/Davdroid information loss.
We are creating an association to provide Nextcloud/Davdroid for our members so they can use Android without Google. So a problem like this could be worrying!