       dup fileId ?targetFileId?
              Duplicate  an open file.  A new file id is opened that addresses
              the same file as fileId.

              If targetFileId is specified, the the file is dup to this speci-
              fied  file  id.  Normally this is stdin, stdout, or stderr.  The
              dup command will handle flushing output and closing  this  file.
              The  new  file  will be buffered, if its needs to be unbuffered,
              use the fcntl command to set it unbuffered.

              If fileId is a number rather than a Tcl file id,  then  the  dup
              command  will  bind that file to a Tcl file id.  This is usedful
              for accessing files that are passed  from  the  parent  process.
              The argument ?targetFileId? is not valid with this operation.

              On  Windows 95/NT, only stdin, stdout, or stderr or a non-socket
              file handle number maybe specified for  targetFileId.   The  dup
              command does not work on sockets on Windows 95/NT.
