I know you're asking for a regexp, but here's another way to do it that will give you an intermediate count of how many dupes; 
my $s = "#tag1 #tag2 #tag3 #tag1";

print qq{Original: $s\n};
my ($x, $d);
map { ++$x->{$_}; $d->{$_}=$x->{$_} if $x->{$_} > 1 } split /\s+/, $s;
printf qq{Uniq: %s\n}, join(' ', sort keys %$x);
printf qq{Dupes found for: %s\n}, join(', ', sort keys %$d);

[download]
Output: 
Original: #tag1 #tag2 #tag3 #tag1
Uniq: #tag1 #tag2 #tag3
Dupes found for: #tag1

[download]
I am sure this can be shrunk down quite a bit, but I tried to strike a balance between terseness and readability. You can inspect $x for counts.

In reply to Re: Regex: remove non-adjacent duplicate hashtags by perlfan
in thread Regex: remove non-adjacent duplicate hashtags by element22

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