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Your contacts can be kept as long as you login into the google accounts under other accounts and sync the contact list of your Google account to your Mi phone. This is one of the most efficient way to get your contacts to remain even after signing out of gmail.
As for number 2, you will have to do it individually
thnkU 4 d reply. does this mean that when I logout from Gmail, the contacts will also disappear? but that's what I want to resolve.. Any method to keep contacts in phone list even after I logout Gmail?
In MIUI, you can store contacts either online or offline.
Google Account or Mi Cloud
If you are logged it to Xiaomi account from your device, you'll not see offline option while storing contacts.
1) Export your contacts from Google to local storage. Log out from Google & Xiaomi account. Import the contacts from local storage.
2) In contact menu, there is an option to merge duplicate contacts, use that.