diff --git a/test/quantization/test_quant_api.py b/test/quantization/test_quant_api.py
index 506cec9dea..e530babdb9 100644
--- a/test/quantization/test_quant_api.py
+++ b/test/quantization/test_quant_api.py
@@ -1178,6 +1178,13 @@ def __init__(self):
         assert isinstance(m.nested.linear.weight, AffineQuantizedTensor)
         assert isinstance(m.linear1.weight, AffineQuantizedTensor)
 
+    def test_fqn_config_module_config_and_fqn_config_both_specified(self):
+        with self.assertRaises(ValueError):
+            FqnToConfig(
+                fqn_to_config={"test": Float8WeightOnlyConfig()},
+                module_fqn_to_config={"test2": Float8WeightOnlyConfig()},
+            )
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/torchao/quantization/quant_api.py b/torchao/quantization/quant_api.py
index 09c2edcd9f..425af1feb9 100644
--- a/torchao/quantization/quant_api.py
+++ b/torchao/quantization/quant_api.py
@@ -2466,6 +2466,15 @@ class FqnToConfig(AOBaseConfig):
     def __post_init__(self):
         torch._C._log_api_usage_once("torchao.quantization.FqnToConfig")
 
+        if (
+            len(self.fqn_to_config) > 0
+            and len(self.module_fqn_to_config) > 0
+            and self.fqn_to_config != self.module_fqn_to_config
+        ):
+            raise ValueError(
+                "`fqn_to_config` and `module_fqn_to_config` are both specified and are not equal!"
+            )
+
         # This code handles BC compatibility with `ModuleFqnToConfig`. It ensures that `self.module_fqn_to_config` and `self.fqn_to_config` share the same object.
         if len(self.module_fqn_to_config) > 0 and len(self.fqn_to_config) == 0:
             self.fqn_to_config = self.module_fqn_to_config
@@ -2479,6 +2488,18 @@ def __post_init__(self):
                 "Config Deprecation: _default is deprecated and will no longer be supported in a future release. Please see https://github.com/pytorch/ao/issues/3229 for more details."
             )
 
+    def __str__(self):
+        return "\n".join(
+            [
+                "FqnToConfig({",
+                *(
+                    f"  '{key}':\n    {value},"
+                    for key, value in self.fqn_to_config.items()
+                ),
+                "})",
+            ]
+        )
+
 
 # maintain BC
 ModuleFqnToConfig = FqnToConfig