diff --git a/src/lean_spec/node/validator/registry.py b/src/lean_spec/node/validator/registry.py
index d3302ef9a..c33364372 100644
--- a/src/lean_spec/node/validator/registry.py
+++ b/src/lean_spec/node/validator/registry.py
@@ -214,6 +214,17 @@ def from_yaml(
                 )
                 continue
 
+            # The attestation and proposal keys must be distinct.
+            # A validator signs a proposal and an attestation in the same slot.
+            # The signature scheme is a stateful one-time signature.
+            # Sharing one key across both roles reuses one-time state and breaks it.
+            # Compare the public keys so the secret bytes stay untouched.
+            if manifest_entry.attestation_public_key_hex == manifest_entry.proposal_public_key_hex:
+                raise ValueError(
+                    "Attestation and proposal keys must differ for validator "
+                    f"{validator_index}, but the manifest assigns the same key to both"
+                )
+
             # Decode the attestation key from its SSZ file.
             attestation_key_path = manifest_directory / manifest_entry.attestation_private_key_file
             try:
diff --git a/tests/cli/test_bootstrap.py b/tests/cli/test_bootstrap.py
index 8fe52b511..610bafa9d 100644
--- a/tests/cli/test_bootstrap.py
+++ b/tests/cli/test_bootstrap.py
@@ -118,6 +118,7 @@ def validator_keys_directory(tmp_path: Path) -> Path:
 
     # Manifest carries one validator with placeholder public keys.
     # The loader does not verify the public_keys against the secret keys here.
+    # The two keys still differ, as the loader requires distinct signing roles.
     manifest = hash_signature_directory / "validator-keys-manifest.yaml"
     manifest.write_text(
         "key_scheme: SIGTopLevelTargetSumLifetime32Dim64Base8\n"
@@ -130,7 +131,7 @@ def validator_keys_directory(tmp_path: Path) -> Path:
         "validators:\n"
         "  - index: 0\n"
         f"    attestation_public_key_hex: '0x{'00' * 52}'\n"
-        f"    proposal_public_key_hex: '0x{'00' * 52}'\n"
+        f"    proposal_public_key_hex: '0x{'00' * 51}ee'\n"
         "    attestation_private_key_file: att_key_0.ssz\n"
         "    proposal_private_key_file: prop_key_0.ssz\n"
     )
diff --git a/tests/node/validator/test_registry.py b/tests/node/validator/test_registry.py
index e51f286de..5ac8e77b2 100644
--- a/tests/node/validator/test_registry.py
+++ b/tests/node/validator/test_registry.py
@@ -60,11 +60,14 @@ def _minimal_manifest_dict(
 
 
 def _manifest_entry_dict(index: int, suffix: str = "") -> dict[str, object]:
-    """Return a manifest entry dict for a validator at the given index."""
+    """Return a manifest entry dict for a validator at the given index.
+
+    The attestation and proposal public keys differ, as the loader requires.
+    """
     return {
         "index": index,
         "attestation_public_key_hex": "0x" + f"{index:02d}" * 52,
-        "proposal_public_key_hex": "0x" + f"{index:02d}" * 52,
+        "proposal_public_key_hex": "0x" + f"{index:02d}" * 51 + "ee",
         "attestation_private_key_file": f"att_key_{index}{suffix}.ssz",
         "proposal_private_key_file": f"prop_key_{index}{suffix}.ssz",
     }
@@ -168,14 +171,14 @@ def test_from_yaml_file_parses_validators_list(self, tmp_path: Path) -> None:
             ValidatorManifestEntry(
                 index=ValidatorIndex(0),
                 attestation_public_key_hex=Bytes52("0x" + "00" * 52),
-                proposal_public_key_hex=Bytes52("0x" + "00" * 52),
+                proposal_public_key_hex=Bytes52("0x" + "00" * 51 + "ee"),
                 attestation_private_key_file="att_key_0.ssz",
                 proposal_private_key_file="prop_key_0.ssz",
             ),
             ValidatorManifestEntry(
                 index=ValidatorIndex(1),
                 attestation_public_key_hex=Bytes52("0x" + "01" * 52),
-                proposal_public_key_hex=Bytes52("0x" + "01" * 52),
+                proposal_public_key_hex=Bytes52("0x" + "01" * 51 + "ee"),
                 attestation_private_key_file="att_key_1.ssz",
                 proposal_private_key_file="prop_key_1.ssz",
             ),
@@ -506,6 +509,34 @@ def test_corrupt_proposal_key_file_raises(self, tmp_path: Path, km: XmssKeyManag
             "Failed to load proposal key for validator 0: PRFKey: expected 32 bytes, got 13"
         )
 
+    def test_same_key_for_both_roles_raises(self, tmp_path: Path) -> None:
+        """A manifest assigning one key to both roles is rejected before any file is read."""
+        validators_file = tmp_path / "validators.yaml"
+        validators_file.write_text(yaml.dump({"node_0": [0]}))
+
+        same_key_entry = {
+            "index": 0,
+            "attestation_public_key_hex": "0x" + "cd" * 52,
+            "proposal_public_key_hex": "0x" + "cd" * 52,
+            "attestation_private_key_file": "att_key_0.ssz",
+            "proposal_private_key_file": "prop_key_0.ssz",
+        }
+        manifest_file = tmp_path / "manifest.yaml"
+        _write_manifest(manifest_file, [same_key_entry])
+
+        # No key files are written on purpose.
+        # The check must fire before any decode is attempted.
+        with pytest.raises(ValueError) as exception_info:
+            ValidatorRegistry.from_yaml(
+                node_id="node_0",
+                validators_path=validators_file,
+                manifest_path=manifest_file,
+            )
+        assert str(exception_info.value) == (
+            "Attestation and proposal keys must differ for validator 0, "
+            "but the manifest assigns the same key to both"
+        )
+
     def test_only_assigned_node_keys_are_loaded(self, tmp_path: Path, km: XmssKeyManager) -> None:
         """Keys for validators belonging to other nodes are never touched."""
         validators_file = tmp_path / "validators.yaml"