diff --git a/knockr.go b/knockr.go
index 0f99a9f..48450e5 100644
--- a/knockr.go
+++ b/knockr.go
@@ -130,6 +130,7 @@ func gateway_handler(c net.Conn) {
 func add_to_whitelist(addr string) {
 	// Add the specified address to the whitelist
 	if ! is_whitelisted(addr) {
+		fmt.Println("[OK ] Add ", addr, " to whitelist")
 		update_whitelist_time(addr)
 	}
 }
@@ -159,7 +160,10 @@ func is_whitelisted(addr string) bool {
 
 func add_to_blacklist(addr string) {
 	// Add specified address to blacklist
-	blacklist = append(blacklist, addr)
+	if ! is_blacklisted(addr) {
+		fmt.Println("[OK ] Add ", addr, " to blacklist")
+		blacklist = append(blacklist, addr)
+	}
 }
 
 func is_blacklisted(addr string) bool {