Author: Andreas Beckmann <anbe@debian.org>
Description: do not create duplicate entries in /etc/shells

--- a/update-shells
+++ b/update-shells
@@ -106,8 +106,10 @@ while IFS= read -r line; do
 	if [ -z "$shell" ] ||
 			hashset_contains "$PKG_SHELLS" "$shell" ||
 			! hashset_contains "$STATE_SHELLS" "$shell"; then
-		echo "$line" >> "$NEW_ETC_FILE"
-		ETC_SHELLS="$ETC_SHELLS$shell#"
+		if [ -z "$shell" ] || ! hashset_contains "$ETC_SHELLS" "$shell"; then
+			echo "$line" >> "$NEW_ETC_FILE"
+			ETC_SHELLS="$ETC_SHELLS$shell#"
+		fi
 	else
 		log "removing shell $shell"
 	fi