When you create users with system-level properties, those users are available to all databases in the system.
You set the value of this system-wide property once for each user, so you can set it several times. To delete a user, remove that user from the file.
You can define this property in the usual ways -- typically in the derby.properties file. For more information about setting properties, see Working with Derby properties.
Here is a sample excerpt from the derby.properties file:
# Users definition
#
derby.user.sa=derbypass
derby.user.mary=little7xylamb