perl -MList::Util=uniq -ple '
  s/
     ^      # Beginning of line (BOL).
     \w+    # Any "words".
     \s+    # Any whitespace (like tabs).
     \K     # "Keep" whats to the left.
     (\S+)  # Capture and replace next non whitespace (words).
   /
     join "-",   # 4: n1-n2-n3
     uniq        # 3: [ "n1", "n2", "n3" ]
     split "-",  # 2: [ "n1", "n2", "n1", "n3" ]
     $1          # 1: n1-n2-n1-n3
   /xe           # Freespace regex and eval replacement.
' in