Yes, only one person can be entering adjustments at a time.

In fact, when users are running the same application from the server, the second user will get a message that they can't access the Adjustment entry function because another user is currently there. This protection is being circumvented by having multiple installations of the program being run locally and accessing the same company data.

You should be aware that data corruption is a possibility when applications are configured in this manner. Rather, the application could be installed on a terminal server and all users from the different location would run the same program files.