diff --git a/awscli/botocore/awsrequest.py b/awscli/botocore/awsrequest.py
index f7a18a3..bf1ca13 100644
--- a/awscli/botocore/awsrequest.py
+++ b/awscli/botocore/awsrequest.py
@@ -14,6 +14,7 @@
 import functools
 import io
 import logging
+from collections.abc import Mapping
 import socket
 import sys
 
@@ -24,6 +25,7 @@ from botocore.compat import (
     HTTPResponse,
     MutableMapping,
     urlencode,
+    urlparse,
     urlsplit,
     urlunsplit,
 )
@@ -375,8 +377,14 @@ class AWSRequestPreparer:
     def _prepare_url(self, original):
         url = original.url
         if original.params:
-            params = urlencode(list(original.params.items()), doseq=True)
-            url = f'{url}?{params}'
+            url_parts = urlparse(url)
+            delim = '&' if url_parts.query else '?'
+            if isinstance(original.params, Mapping):
+                params_to_encode = list(original.params.items())
+            else:
+                params_to_encode = original.params
+            params = urlencode(params_to_encode, doseq=True)
+            url = delim.join((url, params))
         return url
 
     def _prepare_headers(self, original, prepared_body=None):