diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index 595b96d7..b3ac035b 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -169,6 +169,8 @@ class KeyPairBackend(object):
         super(KeyPairBackend, self).__init__()
 
     def create_key_pair(self, name):
+        if name in self.keypairs:
+            raise InvalidIdError(name)
         self.keypairs[name] = keypair = random_key_pair()
         keypair['name'] = name
         return keypair
diff --git a/moto/ec2/responses/key_pairs.py b/moto/ec2/responses/key_pairs.py
index 30cd2dd6..9afabda8 100644
--- a/moto/ec2/responses/key_pairs.py
+++ b/moto/ec2/responses/key_pairs.py
@@ -1,14 +1,21 @@
 from jinja2 import Template
 from moto.core.responses import BaseResponse
 from moto.ec2.models import ec2_backend
+from moto.ec2.exceptions import InvalidIdError
 
 
 class KeyPairs(BaseResponse):
 
     def create_key_pair(self):
-        name = self.querystring.get('KeyName')[0]
-        template = Template(CREATE_KEY_PAIR_RESPONSE)
-        return template.render(**ec2_backend.create_key_pair(name))
+        try:
+            name = self.querystring.get('KeyName')[0]
+            keypair = ec2_backend.create_key_pair(name)
+        except InvalidIdError as exc:
+            template = Template(CREATE_KEY_PAIR_INVALID_NAME)
+            return template.render(keypair_id=exc.id), dict(status=400)
+        else:
+            template = Template(CREATE_KEY_PAIR_RESPONSE)
+            return template.render(**keypair)
 
     def delete_key_pair(self):
         raise NotImplementedError('KeyPairs.delete_key_pair is not yet implemented')
@@ -36,3 +43,8 @@ CREATE_KEY_PAIR_RESPONSE = """<CreateKeyPairResponse xmlns="http://ec2.amazonaws
    <keyMaterial>{{ material }}
     </keyMaterial>
 </CreateKeyPairResponse>"""
+
+
+CREATE_KEY_PAIR_INVALID_NAME = """<?xml version="1.0" encoding="UTF-8"?>
+<Response><Errors><Error><Code>InvalidKeyPair.Duplicate</Code><Message>The keypair '{{ keypair_id }}' already exists.</Message></Error></Errors><RequestID>f4f76e81-8ca5-4e61-a6d5-a4a96EXAMPLE</RequestID></Response>
+"""