Up to Toad 11.0, you could compare a table to itself using Data Compare. However, beginning Toad 11.5, if you try to compare a table to itself(same database/schema), the following information is returned to you;
"You must select two different objects to continue".
Functionality was disabled in Toad 11.5.
Create a view for the table and use it to compare the table.
Enhancement request CR100849 has been submitted to Development for consideration in a future release of Toad for Oracle.