#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11145665
use warnings;

$_ = "#tag1 #tag2 #tag3 #tag1\n";
print "                  original:  $_";
s/(#\w{2,})\b\h*(?=.*\1\b)//g;
print " keeping only the last one:  $_";

$_ = "#tag1 #tag2 #tag3 #tag1\n";
1 while s/(#\w{2,})\b.*\K\1\b\h*//;
print "keeping only the first one:  $_";
[download]

Outputs:

                  original:  #tag1 #tag2 #tag3 #tag1
 keeping only the last one:  #tag2 #tag3 #tag1
keeping only the first one:  #tag1 #tag2 #tag3
[download]