The standard only permits one DISTINCT clause in a select
  expression

  Explanation:  You entered a select expression which either has
  two aggregate functions with DISTINCT clauses, or has a DISTINCT
  clause for the entire select list and a DISTINCT in an aggregate
  function.  The ANSI and ISO SQL standards do not support
  multiple distinct clauses.

  User Action:  If it is important to have only standard features
  in your program, you should remove one of the distinct clauses.