You'll need a way to count the matching values.
Here's one way:
Define a "self join" based on that field.
In the relationships graph, find the Table (table occurrence actually) box for this layout. Let's say it's labeled "YourTable"
Select it and click the button with 2 plus signs to make a new table occurrence that points to the same data source table, unless you change the name, it'll be named YourTable 2.
Drag from BPI in YourTable to the same field in YourTable 2.
Click OK to dismiss Manage | Database.
In your conditional format, this expression:
Count ( YourTable 2::BPI ) > 1
Can be used to change the text or fill color to highlight duplicate values.