To unmerge incorrectly merged contacts, select the contact and click on the three dots in the upper right corner to select View History. Find when the invalid merge has happened, then select Unmerge next to the incorrectly merged contacts.
You can also unmerge contacts in the History section of your Web app. Simply click 'unmerge' in the same row as the merged contacts.
This will revert the invalid merges and will also restore the contacts to their original states. It also will prevent these contacts from getting merged together again.