diff --git a/app/node_replace_manager.py b/app/node_replace_manager.py
index d9aab5b22..72e8ac2b1 100644
--- a/app/node_replace_manager.py
+++ b/app/node_replace_manager.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import logging
+
 from aiohttp import web
 
 from typing import TYPE_CHECKING, TypedDict
@@ -31,8 +33,22 @@ class NodeReplaceManager:
         self._replacements: dict[str, list[NodeReplace]] = {}
 
     def register(self, node_replace: NodeReplace):
-        """Register a node replacement mapping."""
-        self._replacements.setdefault(node_replace.old_node_id, []).append(node_replace)
+        """Register a node replacement mapping.
+
+        Idempotent: if a replacement with the same (old_node_id, new_node_id)
+        is already registered, the duplicate is ignored. This prevents stale
+        entries from accumulating when custom nodes are reloaded in the same
+        process (e.g. via ComfyUI-Manager).
+        """
+        existing = self._replacements.setdefault(node_replace.old_node_id, [])
+        for entry in existing:
+            if entry.new_node_id == node_replace.new_node_id:
+                logging.debug(
+                    "Node replacement %s -> %s already registered, ignoring duplicate.",
+                    node_replace.old_node_id, node_replace.new_node_id,
+                )
+                return
+        existing.append(node_replace)
 
     def get_replacement(self, old_node_id: str) -> list[NodeReplace] | None:
         """Get replacements for an old node ID."""
diff --git a/tests-unit/app_test/node_replace_manager_test.py b/tests-unit/app_test/node_replace_manager_test.py
new file mode 100644
index 000000000..8a3fd18bb
--- /dev/null
+++ b/tests-unit/app_test/node_replace_manager_test.py
@@ -0,0 +1,90 @@
+"""Tests for NodeReplaceManager registration behavior."""
+import importlib
+import sys
+import types
+
+import pytest
+
+
+@pytest.fixture
+def NodeReplaceManager(monkeypatch):
+    """Provide NodeReplaceManager with `nodes` stubbed.
+
+    `app.node_replace_manager` does `import nodes` at module level, which pulls in
+    torch + the full ComfyUI graph. register() doesn't actually need it, so we
+    stub `nodes` per-test (via monkeypatch so it's torn down) and reload the
+    module so it picks up the stub instead of any cached real import.
+    """
+    fake_nodes = types.ModuleType("nodes")
+    fake_nodes.NODE_CLASS_MAPPINGS = {}
+    monkeypatch.setitem(sys.modules, "nodes", fake_nodes)
+    monkeypatch.delitem(sys.modules, "app.node_replace_manager", raising=False)
+    module = importlib.import_module("app.node_replace_manager")
+    yield module.NodeReplaceManager
+    # Drop the freshly-imported module so the next test (or a later real import
+    # of `nodes`) starts from a clean slate.
+    sys.modules.pop("app.node_replace_manager", None)
+
+
+class FakeNodeReplace:
+    """Lightweight stand-in for comfy_api.latest._io.NodeReplace."""
+    def __init__(self, new_node_id, old_node_id, old_widget_ids=None,
+                 input_mapping=None, output_mapping=None):
+        self.new_node_id = new_node_id
+        self.old_node_id = old_node_id
+        self.old_widget_ids = old_widget_ids
+        self.input_mapping = input_mapping
+        self.output_mapping = output_mapping
+
+
+def test_register_adds_replacement(NodeReplaceManager):
+    manager = NodeReplaceManager()
+    manager.register(FakeNodeReplace(new_node_id="NewNode", old_node_id="OldNode"))
+    assert manager.has_replacement("OldNode")
+    assert len(manager.get_replacement("OldNode")) == 1
+
+
+def test_register_allows_multiple_alternatives_for_same_old_node(NodeReplaceManager):
+    """Different new_node_ids for the same old_node_id should all be kept."""
+    manager = NodeReplaceManager()
+    manager.register(FakeNodeReplace(new_node_id="AltA", old_node_id="OldNode"))
+    manager.register(FakeNodeReplace(new_node_id="AltB", old_node_id="OldNode"))
+    replacements = manager.get_replacement("OldNode")
+    assert len(replacements) == 2
+    assert {r.new_node_id for r in replacements} == {"AltA", "AltB"}
+
+
+def test_register_is_idempotent_for_duplicate_pair(NodeReplaceManager):
+    """Re-registering the same (old_node_id, new_node_id) should be a no-op."""
+    manager = NodeReplaceManager()
+    manager.register(FakeNodeReplace(new_node_id="NewNode", old_node_id="OldNode"))
+    manager.register(FakeNodeReplace(new_node_id="NewNode", old_node_id="OldNode"))
+    manager.register(FakeNodeReplace(new_node_id="NewNode", old_node_id="OldNode"))
+    assert len(manager.get_replacement("OldNode")) == 1
+
+
+def test_register_idempotent_preserves_first_registration(NodeReplaceManager):
+    """First registration wins; later duplicates with different mappings are ignored."""
+    manager = NodeReplaceManager()
+    first = FakeNodeReplace(
+        new_node_id="NewNode", old_node_id="OldNode",
+        input_mapping=[{"new_id": "a", "old_id": "x"}],
+    )
+    second = FakeNodeReplace(
+        new_node_id="NewNode", old_node_id="OldNode",
+        input_mapping=[{"new_id": "b", "old_id": "y"}],
+    )
+    manager.register(first)
+    manager.register(second)
+    replacements = manager.get_replacement("OldNode")
+    assert len(replacements) == 1
+    assert replacements[0] is first
+
+
+def test_register_dedupe_does_not_affect_other_old_nodes(NodeReplaceManager):
+    manager = NodeReplaceManager()
+    manager.register(FakeNodeReplace(new_node_id="NewA", old_node_id="OldA"))
+    manager.register(FakeNodeReplace(new_node_id="NewA", old_node_id="OldA"))
+    manager.register(FakeNodeReplace(new_node_id="NewB", old_node_id="OldB"))
+    assert len(manager.get_replacement("OldA")) == 1
+    assert len(manager.get_replacement("OldB")) == 1