Unable to remove duplicates in Contacts.app

I have encountered an interesting situation. I installed High Sierra on a Mac that also has El Cap on a separate volume.

Every time I switched to El Cap and then back to High Sierra, High Sierra would forget that it had been connected to iCloud, meaning that things like iMessages and contacts would be redownloaded.

I suspect it is this that has caused me to have lots and lots of duplicate entries in Contacts.app.

There are no duplicates on either my Macbook or in iCloud.

I've run Look for duplicates in the Contacts.app several times and it now reports that there are none.

But there are lots and lots. What can I do to remove them?

Thank you in advance

Philip