Where I work many people use the same computer on the same account. We now use the shell instead of the GUI just for convenience.
The first time someone committed it
It seems that users credentials are being cached.
Go to your project, open .git/config and remove the lines:
[credential]
helper = store
# or helper=cache
Check git-credential-store for more information.
Edit: OP mentions in the comments that this is happening in Windows, for which, the other answer is more relevant for removing the credentials from the Control Panel.