To elaborate on Amal's response above:
With the radio button list highlighted on the Hierarchy tab, look at the Binding tab on the Object palette on which you should see a box with two columns.
One column for "Value" the other for "Item".
Each value represents one radio button included in the radio button group.
The error message occurs if any of the values for the items listed in this box are the same.
Change the values in this box to something like 1, 2, 3, etc. by highlighting a value, clicking F2 and entering new value.