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/credential.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"crypto/rand"
 	"encoding/base64"
+	"errors"
 	"os"
 
 	"github.com/minio/mc/pkg/console"
@@ -75,6 +76,16 @@ type credential struct {
 	secretKeyHash []byte
 }
 
+func (c *credential) Validate() error {
+	if !isAccessKeyValid(c.AccessKey) {
+		return errors.New("Invalid access key")
+	}
+	if !isSecretKeyValid(c.SecretKey) {
+		return errors.New("Invalid secret key")
+	}
+	return nil
+}
+
 // Generate a bcrypt hashed key for input secret key.
 func mustGetHashedSecretKey(secretKey string) []byte {
 	hashedSecretKey, err := bcrypt.GenerateFromPassword([]byte(secretKey), bcrypt.DefaultCost)