you can change
COUNT(c1 + c2 )

into 

count(*)  && just counting the number of records anyway

>DELETE FROM Schnaps;
> WHERE i1 IN ;
> (;
>  SELECT S.i1 ;
>   FROM Schnaps AS S;
>   INNER JOIN;
>    ( ;
>    SELECT c3, MIN(i1) AS i1;
>     FROM Schnaps ;
>     GROUP BY  1 ;
>     HAVING ( COUNT(c1 + c2 ) > 1) ;
>    ) AS AA ;
>   ON S.c3 == AA.c3;
>   AND S.i1>AA.i1;
> )
>