You can do the following to check uniqueness of the data:
1) Add an attribute to your business object that will store a unique identifier of the record. When creating data on a PDA make sure that this data is initialised appropriately.
2) Add a rule that will reject a new record if it is a duplicate of the existing record:
If EXISTS MyObject WHERE (MyObject.UniqueDataID = ThisMyObject.UniqueDataID AND MyObject.ID <> ThisMyObject.ID) Then
REPORT ERROR 'Data already exists'
3) If you are going to be importing data using CSV files make sure that the "validate data" option is enabled - in this case duplicate records will be ignored.