Is there a way to make it so only one person can have a specific form open at once?
I'm thinking maybe setting up a public module with a value that is set to "1" on the On Open event of the form and set to "0" on the On Close event, then check this value when the form opens (before setting the value to zero) and close it if necessary. But I'm not exactly sure how to do this or even if it'll work.
To allow two or more users to open the same database mdb file is not a good idea in the first place. Better to have separate front end mdb files for each user which look (link) to the communal data in the back end mdb file.
If the reason for your question is to stop several people having access to the same data at the same time then look at the record locking options (in the form design).