How to prevent iCloud repeatedly duplicating my Contacts?

My desktop (via Outlook) and my iPad and my iPhone are all backed up via Apple's iCloud. But my Contacts repeatedly duplicate so that I soon have several of many of them. How can I prevent that? I do have more than one account (email address) on my iPad and iPhone, but only one of them is set to sync Contacts via iCloud.

Once you have put anything on to the internet it is on a computer not controlled by you. There's no way you can control what someone else chooses to do on their computer.

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