Databases and database applications don't begin with Forms -- they begin
with User Requirements and the Data needed to fill those requirements.
Without some idea of what you have, and what you are trying to accomplish,
it's possible that we might (correctly in a technical sense) answer your
questions and not be much help at all.

You can ensure that an entry into a field doesn't duplicate an existing
entry, if you make that field an index on the table, and do not allow
duplicates (this is done in table design); it can also be handled in code in
form events.

How are the users going to update the data displayed in the list box?  The
Query used as Row Source for the list box will be executed when the Form is
opened;  you can refresh the List Box by executing its Requery method.
Generally, if I am doing something similar to what you describe, I'll do it
with a Combo Box -- in that case, nothing need be done to ensure that the
new value is used.

If you can clarify, you might get some more specific answers.

Larry Linson
Microsoft Office Access MVP