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                                                                                                                               

##</code><code>##

  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

##</code><code>##

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

  DB<82> $_=$r; s/(#\S+\s?)(?=(.+\1))(?{DEB})//g ;say "fin: $_"                                                               
#1 #2 #3 #1 #3 #2 #3 #2 (#1 ) - (#2 #3 #1 ) pos:4                                                                            
#1 #2 #3 #1 #3 #2 #3 #2 (#2 ) - (#3 #1 #3 #2 ) pos:7                                                                         
#1 #2 #3 #1 #3 #2 #3 #2 (#3 ) - (#1 #3 #2 #3 ) pos:10                                                                        
#1 #2 #3 #1 #3 #2 #3 #2 (#3 ) - (#2 #3 ) pos:16                                                                              
#1 #2 #3 #1 #3 #2 #3 #2 (#2) - ( #3 #2) pos:18                                                                              
fin:  #1  #3 #2                                                                                                                                                                                                                                             

DB<83>