diff --git a/IkiWiki/Plugin/po.pm b/IkiWiki/Plugin/po.pm
index d920d3648..ab39cca12 100644
--- a/IkiWiki/Plugin/po.pm
+++ b/IkiWiki/Plugin/po.pm
@@ -152,7 +152,7 @@ sub checkconfig () {
 	if (ref $config{po_slave_languages} eq 'ARRAY') {
 		foreach my $pair (@{$config{po_slave_languages}}) {
 			my ($code, $name)=splitlangpair($pair);
-			if (defined $code) {
+			if (defined $code && ! exists $slavelanguages{$code}) {
 				push @slavelanguages, $code;
 				$slavelanguages{$code} = $name;
 			}