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