Are you saying currently you have a table that doesn't have a primary
key or unique value in one of the fields.
If this is true try adding a new field with AutoNum to increment automatically.
Or you want a unique value coming from a expression / calculation from other field values. If this is what you want you can do is
set an expression from a control/text box prop. in a Form using default value.
Or in a form, under design view, try to create a function/vba code in event tab properties like After Update. This could make you customize
your values to be unique.
Hope this helps.