       1mdup 4m22mfileId24m ?4mtargetFileId24m?
              Duplicate  an open file.  A new file id is opened that addresses
              the same file as 4mfileId24m.

              If 4mtargetFileId24m is specified, the the file is dup to this speci-
              fied  file  id.  Normally this is 1mstdin22m, 1mstdout22m, or 1mstderr22m.  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 1mfcntl 22mcommand to set it unbuffered.

              If 4mfileId24m is a number rather than a Tcl file id,  then  the  1mdup0m
              command  will  bind  that file to a Tcl file id.  This is useful
              for accessing files that are passed  from  the  parent  process.
              The argument ?4mtargetFileId24m? is not valid with this operation.

              On  1mWindows22m,  only 1mstdin22m, 1mstdout22m, or 1mstderr 22mor a non-socket file
              handle number maybe specified for 4mtargetFileId24m.  The 1mdup 22mcommand
              does not work on sockets on 1mWindows22m.
