Dorod(means hi)

in access i have many problem, i wrote the following
1- i have a combo box that control source refer to a field in table, when i type a duplicate value in combo box access shows error. i want when type duplicate value instead of a error a message box show up. how can do it?

2- how can i for a special form disable design view?

3- how can i for a access file disable navigation pane not through file>options>current database>display navigation pane ?