diff --git a/zmldapsync/zmldapsync.pl b/zmldapsync/zmldapsync.pl
index 6169d6a..4a33680 100644
--- a/zmldapsync/zmldapsync.pl
+++ b/zmldapsync/zmldapsync.pl
@@ -9,6 +9,7 @@ use Getopt::Long;
 use Data::UUID;
 use String::ShellQuote qw(shell_quote);
 use Array::Diff;
+use List::MoreUtils qw(uniq);
 use Hash::Merge::Simple qw(merge);
 use Text::Unidecode;
 use Email::MIME;
@@ -388,7 +389,7 @@ DOMAIN: foreach my $domain ( keys $conf->{domains} ) {
              @{ $ext_users->{$user}->{$conf->{domains}->{$domain}->{users}->{$mail_attr}} };
     }
 
-    @ext_aliases = sort @ext_aliases;
+    @ext_aliases = sort uniq @ext_aliases;
 
     foreach my $alias ( @ext_aliases ) {
       next if ( not alias_matches_domain( $alias, $domain_entry ) );
@@ -630,6 +631,7 @@ DOMAIN: foreach my $domain ( keys $conf->{domains} ) {
         push @ext_aliases,
                @{ $ext_groups->{$group}->{$conf->{domains}->{$domain}->{groups}->{$mail_attr}} };
       }
+      @ext_aliases = sort uniq @ext_aliases;
       foreach my $alias ( @ext_aliases ) {
         next if ( not alias_matches_domain( $alias, $domain_entry ) );
         next if ( grep { $alias eq $_ } @{ $zim_dl->{$group}->{mail} } );