Better EC2 duplicate SG error.

moto/ec2/exceptions.py

@@ -156,6 +156,14 @@ class InvalidPermissionNotFoundError(EC2ClientError):
             "The specified rule does not exist in this security group")
 
 
+class InvalidPermissionDuplicateError(EC2ClientError):
+
+    def __init__(self):
+        super(InvalidPermissionDuplicateError, self).__init__(
+            "InvalidPermission.Duplicate",
+            "The specified rule already exists")
+
+
 class InvalidRouteTableIdError(EC2ClientError):
 
     def __init__(self, route_table_id):

moto/ec2/models.py

@@ -35,6 +35,7 @@ from .exceptions import (
     InvalidSecurityGroupDuplicateError,
     InvalidSecurityGroupNotFoundError,
     InvalidPermissionNotFoundError,
+    InvalidPermissionDuplicateError,
     InvalidRouteTableIdError,
     InvalidRouteError,
     InvalidInstanceIdError,
@@ -1311,7 +1312,7 @@ class SecurityGroup(TaggedEC2Resource):
 
     def add_ingress_rule(self, rule):
         if rule in self.ingress_rules:
-            raise InvalidParameterValueError('security_group')
+            raise InvalidPermissionDuplicateError()
         else:
             self.ingress_rules.append(rule)