I’m unable to merge duplicate contacts on my iPhone

i have the same problem. 'merge all' does not merge any contacts. I can only Merge one contact at a time. With 1,498 duplicates this is a problem.

Apple Support

I created this User Tip to serve and the basic framework for managing and recovering Contacts - Contacts

Contacts can sometimes become scattered or multiplied when syncing or when changing devices. To ensure that contacts are always correct, follow these few simple steps. These steps include a Mac but do not include the steps for a PC. These steps include the use of iCloud and do not require purchasing additional storage. Make sure you back up your devices regularly so you have a back up to restore from if needed and make sure that you are always using the latest software update available for every Apple device you are using.

If you are missing contacts because you recently deleted an email account, add it back until you perform these steps. If you are using iCloud you can also try going to iCloud.com>sign in>account settings>advanced>restore contacts>select from the list of "archive of contacts."

If you do not wish to use iCloud, you can select one of the emails to use instead of iCloud. Having one source for contacts prevents them from being scattered or mismanaged.

Step one

(Skip to step two if you do not own an iPhone/iPad)

On iPhone/iPad

1. Go to settings>mail>accounts>tap any account and turn on contacts (repeat for all accounts).
2. Go to settings>Apple ID (your name at the top>iCloud>iCloud backup - turn on and select "back up now."
3. Go back to settings>Apple ID>iCloud>turn contacts off - choose to keep on device. Do not turn it back on yet!
4. Go to settings>mail>accounts>tap any account except iCloud - turn off contacts (repeat for all accounts).
5. Go to settings>mail>accounts>tap iCloud - turn on contacts. Stop here on the iPhone until you perform the Mac steps.

Step two

(Skip to step three if you do not own a Mac)

On Mac

1. Go to system preferences>internet accounts>tap iCloud account - turn on contacts
2. Go to system preferences>internet accounts> are the contacts turned on for all accounts?
3. Go to system preferences>iCloud>contacts - uncheck that, and then check it again and it will ask to merge. This should force it to merge all the contacts you have for the Mac.
4. Go to system preferences>internet accounts>tap any account that is not iCloud - turn off contacts (repeat for all accounts).
5. Stop there and go to iCloud.com

Step three

On iCloud.com

1. Sign in and select contacts - make sure they are correct.
2. If there are duplicate contacts select the contacts that are duplicate and then go to the gear icon (settings) in the lower left corner. Select delete. Repeat for every duplicate/old contact you no longer need. You can select multiple contacts by using the CTRL+Click method (if you are unfamiliar with that method do not attempt it). Once all the extra contacts are deleted go back to the gear icon (settings) and select refresh contacts.

Step four

On iPhone

1. Go back to settings>Apple ID>iCloud>turn contacts on and merge.
2. Go to settings>Apple ID>iCloud>iCloud backup - turn on

Useful Links

View contacts on iCloud.com - Apple Support

Manage and delete contacts on your iPhone, iPad, or iPod touch - Apple Support

Contacts User Guide for Mac - Apple Support

Set up and use iCloud Contacts - Apple Support

If your iCloud Contacts, Calendars, or Reminders won’t sync - Apple Support

Import contacts from your SIM card to your iPhone - Apple Support

If you see duplicate contacts after setting up iCloud Contacts - Apple Support

That didn't work.

look at my answer.