Fix duplicate custom node import when cross-referenced

When one custom node imports another custom node (e.g., via 'import custom_nodes.other_node'),
the ComfyUI custom node loader would previously load the imported node twice:
1. First time: via the explicit import statement (module name: custom_nodes.other_node)
2. Second time: via the automatic custom node scanner (module name: custom_nodes/other_node)

This caused issues including:
- Module initialization code being executed twice
- Route handlers being registered multiple times
- Potential state inconsistencies and data loss

This fix adds a check before loading a custom node to detect if it's already loaded
in sys.modules under either the standard import name or the path-based name.
If already loaded, it reuses the existing module instead of re-executing it.

Benefits:
- Prevents duplicate initialization of custom nodes
- Allows custom nodes to safely import other custom nodes
- Improves performance by avoiding redundant module loading
- Maintains backward compatibility with existing custom nodes

Also includes linting fix: use lazy % formatting in logging functions.

nodes.py

@@ -2135,18 +2135,42 @@ async def load_custom_node(module_path: str, ignore=set(), module_parent="custom
     elif os.path.isdir(module_path):
         sys_module_name = module_path.replace(".", "_x_")
 
+    # Check if module is already loaded to prevent duplicate imports
+    # This can happen when one custom node imports another custom node
+    standard_module_name = f"{module_parent}.{get_module_name(module_path)}"
+    already_loaded = False
+    module = None
+    
+    if sys_module_name in sys.modules:
+        logging.debug("Custom node %s already loaded, reusing existing module", sys_module_name)
+        module = sys.modules[sys_module_name]
+        already_loaded = True
+    elif standard_module_name in sys.modules:
+        logging.debug("Custom node %s already loaded via standard import, reusing existing module", standard_module_name)
+        module = sys.modules[standard_module_name]
+        # Register the module under sys_module_name as well to avoid future conflicts
+        sys.modules[sys_module_name] = module
+        already_loaded = True
+
     try:
         logging.debug("Trying to load custom node {}".format(module_path))
+        
+        # Determine module_dir regardless of whether module is already loaded
         if os.path.isfile(module_path):
-            module_spec = importlib.util.spec_from_file_location(sys_module_name, module_path)
             module_dir = os.path.split(module_path)[0]
         else:
-            module_spec = importlib.util.spec_from_file_location(sys_module_name, os.path.join(module_path, "__init__.py"))
             module_dir = module_path
+        
+        # Only execute module loading if not already loaded
+        if not already_loaded:
+            if os.path.isfile(module_path):
+                module_spec = importlib.util.spec_from_file_location(sys_module_name, module_path)
+            else:
+                module_spec = importlib.util.spec_from_file_location(sys_module_name, os.path.join(module_path, "__init__.py"))
 
-        module = importlib.util.module_from_spec(module_spec)
-        sys.modules[sys_module_name] = module
-        module_spec.loader.exec_module(module)
+            module = importlib.util.module_from_spec(module_spec)
+            sys.modules[sys_module_name] = module
+            module_spec.loader.exec_module(module)
 
         LOADED_MODULE_DIRS[module_name] = os.path.abspath(module_dir)
 

tests-unit/nodes_test/test_duplicate_import.py

@@ -0,0 +1,126 @@
+"""
+Test for preventing duplicate custom node imports.
+
+This test verifies that when a custom node is imported by another custom node,
+the module loading mechanism correctly detects and reuses the already-loaded module
+instead of loading it again.
+"""
+import sys
+import os
+import pytest
+from unittest.mock import MagicMock, patch
+
+# Mock the required modules before importing nodes
+mock_comfy_api = MagicMock()
+mock_comfy_api.latest.io.ComfyNode = MagicMock
+mock_comfy_api.latest.ComfyExtension = MagicMock
+
+sys.modules['comfy_api'] = mock_comfy_api
+sys.modules['comfy_api.latest'] = mock_comfy_api.latest
+sys.modules['comfy_api.latest.io'] = mock_comfy_api.latest.io
+
+# Mock folder_paths
+mock_folder_paths = MagicMock()
+sys.modules['folder_paths'] = mock_folder_paths
+
+# Mock comfy modules
+sys.modules['comfy'] = MagicMock()
+sys.modules['comfy.model_management'] = MagicMock()
+
+# Now we can import nodes
+import nodes
+
+
+@pytest.mark.asyncio
+async def test_no_duplicate_import_when_already_loaded():
+    """
+    Test that load_custom_node detects and reuses already-loaded modules.
+    
+    Scenario:
+    1. Custom node A is loaded by another custom node (e.g., via direct import)
+    2. ComfyUI's custom node scanner encounters custom node A again
+    3. The scanner should detect that A is already loaded and reuse it
+    """
+    # Create a mock module
+    mock_module = MagicMock()
+    mock_module.NODE_CLASS_MAPPINGS = {}
+    mock_module.WEB_DIRECTORY = None
+    
+    # Simulate that the module was already imported with standard naming
+    module_name = "custom_nodes.test_node"
+    sys.modules[module_name] = mock_module
+    
+    # Track if exec_module is called (should not be called for already-loaded modules)
+    exec_called = False
+    
+    def mock_exec_module(module):
+        nonlocal exec_called
+        exec_called = True
+    
+    # Patch the importlib methods
+    with patch('importlib.util.spec_from_file_location') as mock_spec_func, \
+         patch('importlib.util.module_from_spec') as mock_module_func:
+        
+        mock_spec = MagicMock()
+        mock_spec.loader.exec_module = mock_exec_module
+        mock_spec_func.return_value = mock_spec
+        mock_module_func.return_value = MagicMock()
+        
+        # Create a temporary test directory to simulate the custom node path
+        import tempfile
+        with tempfile.TemporaryDirectory() as tmpdir:
+            test_node_dir = os.path.join(tmpdir, "test_node")
+            os.makedirs(test_node_dir)
+            
+            # Create an __init__.py file
+            init_file = os.path.join(test_node_dir, "__init__.py")
+            with open(init_file, 'w') as f:
+                f.write("NODE_CLASS_MAPPINGS = {}\n")
+            
+            # Attempt to load the custom node
+            # Since we mocked sys.modules with 'custom_nodes.test_node',
+            # the function should detect it and not execute the module again
+            result = await nodes.load_custom_node(test_node_dir)
+            
+            # The function should return True (successful load)
+            assert result == True
+            
+            # exec_module should NOT have been called because module was already loaded
+            assert exec_called == False, "exec_module should not be called for already-loaded modules"
+
+
+@pytest.mark.asyncio
+async def test_load_new_module_when_not_loaded():
+    """
+    Test that load_custom_node properly loads new modules that haven't been imported yet.
+    """
+    import tempfile
+    
+    # Create a temporary test directory
+    with tempfile.TemporaryDirectory() as tmpdir:
+        test_node_dir = os.path.join(tmpdir, "new_test_node")
+        os.makedirs(test_node_dir)
+        
+        # Create an __init__.py file with required attributes
+        init_file = os.path.join(test_node_dir, "__init__.py")
+        with open(init_file, 'w') as f:
+            f.write("NODE_CLASS_MAPPINGS = {}\n")
+        
+        # Clear any existing module with this name
+        sys_module_name = test_node_dir.replace(".", "_x_")
+        if sys_module_name in sys.modules:
+            del sys.modules[sys_module_name]
+        
+        # Load the custom node
+        result = await nodes.load_custom_node(test_node_dir)
+        
+        # Should return True for successful load
+        assert result == True
+        
+        # Module should now be in sys.modules
+        assert sys_module_name in sys.modules
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])
+