Same stream given more than once in PUT/OUTPUT-TO action.
          #7900   Run-time warning
          Same stream given more than once in PUT/OUTPUT-TO action.
Additional Information
 For stream 'X'.
Explanation
A stream may only be specified once in a list of streams. The
following is not allowed because stream foo is already active:
  ELEMENT a                ; <!element a - - (b)*>
    PUT foo "%c"
  
  ELEMENT b
    PUT (#OUTPUT AND foo) "%c"