Duplicate contacts with Multiple iDevices Connected

Working with customer at work. They have 3 devices on the same Apple account. iCloud is on and contacts are syncing. Over the years, using Mobile Iron MDM, They have anywhere from 3 to 5 duplicates per contact. This is Bloating his contacts from 6000 to over 25,000 to 30,000. Merge on the iPhone is not a good solution because it still has all the duplicates. I have downloaded them to a Mac and reduced the duplicates with the contacts app. From this point, What is the best way to remove the contacts on iCloud and replace them with what I have reduced? I tried deleting duplicates manually from iCloud and after a couple of days, they were ALL back. I imagine the other devices synced them back.

Do I need to turn off iCloud sync on the contacts for each device, delete them from each device, delete them from iCloud and finally upload the contacts I have consolidated on the Mac?