fix: reject ambiguous legacy dict-key names in load_state

Distinct dict keys such as "0" and 0 shared the single pre-#1302 name
dict_a['0'], so the legacy fallback applied one saved state to both
parameters. Raise a KeyError when the fallback name is ambiguous.

Author: STiFLeR7

dspy/primitives/base_module.py

@@ -183,6 +183,13 @@ def _apply(module):
                 if name in state:
                     param_state = state[name]
                 elif legacy_name in state:
+                    if legacy_names.count(legacy_name) > 1:
+                        raise KeyError(
+                            f"'{name}': the pre-fix name '{legacy_name}' is ambiguous (it was shared by several "
+                            'dict keys, e.g. "0" and 0, before '
+                            "https://github.com/stanfordnlp/dspy/issues/1302 was fixed), so the saved state cannot "
+                            "be mapped to a single parameter. Re-save the state with the current DSPy version."
+                        )
                     param_state = state[legacy_name]
                 else:
                     raise KeyError(

tests/primitives/test_module.py

@@ -222,6 +222,24 @@ def __init__(self):
     prog.load_state(legacy_state)  # must not raise
 
 
+def test_load_state_rejects_ambiguous_pre_fix_dict_key_names():
+    """
+    Distinct keys "0" and 0 shared the single legacy name "dict_a['0']", so a legacy state cannot say which
+    parameter it belongs to. load_state() must reject it rather than apply one state to both parameters.
+    """
+
+    class Prog(Module):
+        def __init__(self):
+            super().__init__()
+            self.dict_a = {"0": dspy.Predict("question -> answer"), 0: dspy.Predict("question -> answer")}
+
+    prog = Prog()
+    legacy_state = {"dict_a['0']": prog.dict_a["0"].dump_state()}
+
+    with pytest.raises(KeyError, match="ambiguous"):
+        prog.load_state(legacy_state)
+
+
 def test_load_state_raises_clear_error_when_name_missing_in_both_formats():
     """
     A parameter missing under BOTH its current name and its pre-#1302 legacy name is a genuine