iCloud.com - duplicate contacts

Duplicate contacts are viewed in iPhone / iPad if you set up various accounts using gmail , yahoo ....etc and the iPhone / iPad is synced ( signed in with same Apple ID and password on the same network ) Set up and use iCloud Contacts - Apple Support they can be deleted from iPhone or iPad individually Manage and delete contacts on your iPhone, iPad, or iPod touch - Apple Support 

But there is no way to select all duplicate contacts in iPhone as if compared to Mac .

In Mac there is option to merge duplicate contacts as the data base of Mac is larger than an iPhone , however Mac is not synced with other devices .

See the thread https://discussions.apple.com/thread/253109101

Note : when you sign out Apple ID from iPhone / iPad if you forgot to check mark and save copy on device it doesn't matter the contacts and other duplicates contacts that are mirrored from gmail . iCloud , yahoo. ...services used are saved in iCloud server as the contact box was always turned on , once again you sign in with same Apple ID and password on iPhone / iPad all contacts with duplicates will again sync .

Second method if iPhone and only iPad is synced .

To avoid it , disconnect the desired device by just signing out from Apple ID account , then delete the duplicate contacts in that device .

Say if iPhone is signed out delete duplicate contacts from iPad, however the iPad won't be signed out .