John,

It's documented in VFP8 help under Behavior Changes Since Visual FoxPro 7.0 and INFO: SQL SELECT Behavior Changes in Visual FoxPro 8.0 mskb #813361.
Either remove field1 from the column list or add it to the GROUP BY.

>I use this handy dandy sql statement in VFP 7 to find duplicates. For some reason it doesn't work in VFP 8.
>
>The generated error is: SQL: GROUP BY clause is missing or invalid.
>
>SELECT field1, field2, cnt(*) ;
>FROM TableName ;
>GROUP BY field2 ;
>HAVING cnt(*) > 1 ;
>INTO CURSOR Dupes