Teh first option is to create another field.

About the dual: Yes, it's to be created directly in the table. If you have a table you should have added some fields, this fields can be a dimension (segmentates data, like Category, Name) and metrics (calculates values, like sum(Amount), Count(Names))... instead of add a simple field you can edit it to be an expression.  If it is done in a dimension it is called a calculated dimension.

Dual() creates a values wich first parameter is the value shown and the second parameter is the internal value so you can show the same string different times because every string has a different internal value.