After moving from ReactTable 2.2.8 to 3.0.4 the group by functionality has stopped working.
The following warning is displayed in the console log:
Each child in an array should have a unique "key" prop. Check the render method of ReactTable. See http://fb.me/react-warning-keys for more information.
I can confirm that the data does not contain duplicate rows as I added a unique key for each row.
Also tried changing the rowkey option to use the unique key without success.