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 DB<70> $r = " #1 #2 #3 #1 #3 #2 #3 #2"

 DB<72> $_=$r; s/(#\S+\s?)(.+$1)(?{say "$_ ($1) - ($2) pos:",pos($_)})
+/$2/g ;say "fin: $_"
#1 #2 #3 #1 #3 #2 #3 #2 (#1 ) - (#2 #3 #1 #3 #2 #3 #2) pos:24
fin:  #2 #3 #1 #3 #2 #3 #2

  DB<73> $_=$r; s/(#\S+\s?)\K(.+\1)(?{say "$_ ($1) - ($2) pos:",pos($_
+)})//g ;say                                             
#1 #2 #3 #1 #3 #2 #3 #2 (#1 ) - (#2 #3 #1 ) pos:13                    
+                                                       
#1 #2 #3 #1 #3 #2 #3 #2 (#3 ) - (#2 #3 ) pos:22                       
+                                                       
#1 #3 #2

  DB<81> sub DEB {say "$_ ($1) - ($2) pos:",pos($_)}

...
fin:  #1  #3 #2                                                       
+                                                                     
+                                                                     
+                                            

DB<83>