Raise a client error if PutBucketTags request contains duplicate keys

A PutBucketTags request with duplicate keys will raise a ClientError
with code InvalidTag and message 'Cannot provide multiple Tags with the
same key'.

moto/s3/exceptions.py

@@ -178,3 +178,12 @@ class InvalidStorageClass(S3ClientError):
             "InvalidStorageClass",
             "The storage class you specified is not valid",
             *args, **kwargs)
+
+class DuplicateTagKeys(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(DuplicateTagKeys, self).__init__(
+            "InvalidTag",
+            "Cannot provide multiple Tags with the same key",
+            *args, **kwargs)

moto/s3/models.py

@@ -15,7 +15,7 @@ from bisect import insort
 from moto.core import BaseBackend, BaseModel
 from moto.core.utils import iso_8601_datetime_with_milliseconds, rfc_1123_datetime
 from .exceptions import BucketAlreadyExists, MissingBucket, InvalidPart, EntityTooSmall, MissingKey, \
-    InvalidNotificationDestination, MalformedXML, InvalidStorageClass
+    InvalidNotificationDestination, MalformedXML, InvalidStorageClass, DuplicateTagKeys
 from .utils import clean_key_name, _VersionedKeyStore
 
 UPLOAD_ID_BYTES = 43
@@ -773,6 +773,9 @@ class S3Backend(BaseBackend):
         return key
 
     def put_bucket_tagging(self, bucket_name, tagging):
+        tag_keys = [tag.key for tag in tagging.tag_set.tags]
+        if len(tag_keys) != len(set(tag_keys)):
+            raise DuplicateTagKeys()
         bucket = self.get_bucket(bucket_name)
         bucket.set_tags(tagging)