Service request is auto populating with the different user name in requester filed in service portal

[arsh]

There are 2 users with same user names but email ID and user ID is different they can access only for the service portal but when ever the one user is clicking on server request another user is auto populated in the requester field instead of the same user should be auto populated .it's happening for only one user and only in service request; service catalog is working fine and but it's working fine for the service catalog only problem with the service request.and only with one user 

Note: we cannot change the username 

[Mark Manders]

How is the auto-population set? Is that the same on both?

And how are they logging in? Both with the same username, but different passwords? Changing the username would be the best solution here.