diff --git a/moto/cognitoidp/models.py b/moto/cognitoidp/models.py
index 69ab61575..f189f593e 100644
--- a/moto/cognitoidp/models.py
+++ b/moto/cognitoidp/models.py
@@ -1335,6 +1335,8 @@ class CognitoIdpBackend(BaseBackend):
         self.admin_get_user(user_pool_id, username)
 
         for token, token_tuple in list(user_pool.refresh_tokens.items()):
+            if token_tuple is None:
+                continue
             _, username = token_tuple
             if username == username:
                 user_pool.refresh_tokens[token] = None
diff --git a/tests/test_cognitoidp/test_cognitoidp.py b/tests/test_cognitoidp/test_cognitoidp.py
index 13868f358..b44a6ad4a 100644
--- a/tests/test_cognitoidp/test_cognitoidp.py
+++ b/tests/test_cognitoidp/test_cognitoidp.py
@@ -2907,6 +2907,33 @@ def test_admin_user_global_sign_out():
     err["Message"].should.equal("Refresh Token has been revoked")
 
 
+@mock_cognitoidp
+def test_admin_user_global_sign_out_twice():
+    conn = boto3.client("cognito-idp", "us-west-2")
+    result = user_authentication_flow(conn)
+
+    conn.admin_user_global_sign_out(
+        UserPoolId=result["user_pool_id"], Username=result["username"],
+    )
+
+    conn.admin_user_global_sign_out(
+        UserPoolId=result["user_pool_id"], Username=result["username"],
+    )
+
+    with pytest.raises(ClientError) as ex:
+        conn.initiate_auth(
+            ClientId=result["client_id"],
+            AuthFlow="REFRESH_TOKEN",
+            AuthParameters={
+                "REFRESH_TOKEN": result["refresh_token"],
+                "SECRET_HASH": result["secret_hash"],
+            },
+        )
+    err = ex.value.response["Error"]
+    err["Code"].should.equal("NotAuthorizedException")
+    err["Message"].should.equal("Refresh Token has been revoked")
+
+
 @mock_cognitoidp
 def test_admin_user_global_sign_out_unknown_userpool():
     conn = boto3.client("cognito-idp", "us-west-2")