depends on how your user profile service is setup.
look at your hidden user list. any time someone logs to the site for the first time its recorded on that list:
above would be used like so:
look at the list for the offending user, so on my list I could possibly have two ali jafer users (friendly name).
go into central admin and go to user profile service and look up for offending user.
go to active directory and search for offending user.
you need to compare all three to see what is going on. Is it the same account or separate accounts. Specifically on AD look at the samaccount and not the friendly name as that has to be unique.
check on sharepoint user profile service the accounts that have been imported and are upto date:
make sure AD is clean and the user once looked up is setup correctly with one account that they are signing in with or multiple accounts are correct. make sure that the friendly name is different within the AD group if possible else should sign in with the samaccount if possible.
an example of what I mean is:
friendly name: ali jafer
samaccount : ali-jafer
below image shows ad account, the logon name is usually the first and last name (friendly name or user principal name) and the samaccount is marked below.
I could have in theory multiple ali jafer users just like joe blogs or jon smiths but can only have one samaccount that is ali jafer or joe blogs or jon smiths. logging in with that account guarantees its that account that im logging in with.
next is to remove offending user from the hidden sharepoint list. This will not effect the user and will be added back in when he/she signs back into the site. next is to do a full sync on central admin on the user profile service....good to do this out of hours - just set the timer job to run after 1am is fine.... full sync should run once a week usually in the weekends and partial every night.
if for instance the user was to update the details on mysite and you want AD updated then that would need to be setup on central admin user profile service as an export per property look here for more info: