What is the best app for removing duplicate contacts on an android phone?

Check what the source of the contacts is on your setup.

you could be using many sources that replicate.
phone, google account, samsung account, simcard contacts?

while they look the same, and point to the same, if they are stored in different locations, the phone does not see them as the same.
try swithcing to only use one source for contacts and see if that solves your issue.

Identifying the source may help reveal the cause for the duplicates..

I think this is related to syncing and per your suggestion, I have eliminated all but two google accounts that I use regularly. I still need to get rid of hundreds of duplicates and have started doing it manually. That is why I wanted to know if there was a good app that could do the job automatically but wouldn't invade my contacts.

Samsung Android smartphone has a built-in Contacts app which you can merge all your contacts by Number ("telephone number"), Email ("email address") or Name of your contact.

Open your Contacts app, click the Settings button, click "Manage contacts", click "Merge contacts".

Thank you all for your concern and assistance. I apologize for not acknowledging your help until now. Stuff happens.