config: Check for duplicated entries in all scopes (#3872)

Validate Minio config by checking if there is double json key
in any scope level. The returned error contains the json path
to the duplicated key.

cmd/notify-kafka.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 
@@ -39,6 +40,16 @@ type kafkaNotify struct {
 	Topic string `json:"topic"`
 }
 
+func (k *kafkaNotify) Validate() error {
+	if !k.Enable {
+		return nil
+	}
+	if len(k.Brokers) == 0 {
+		return errors.New("No broker specified")
+	}
+	return nil
+}
+
 // kafkaConn contains the active connection to the Kafka cluster and
 // the topic to send event notifications to.
 type kafkaConn struct {