Fix duplicate custom node installation in workflow import

When importing workflows with custom nodes that appear multiple times in the node index (e.g., ComfyUI-Impact-Pack), the same extension was being added to MissingNodes multiple times. This caused duplicate InstallExtensionStep objects to be created, leading to multiple git clone attempts for the same extension. The second clone would fail because the directory already exists.

Added deduplication using a HashSet to track which extension titles have already been added to MissingNodes, ensuring each extension is only installed once.

Author: NeuralFault

StabilityMatrix.Avalonia/ViewModels/Dialogs/OpenArtWorkflowViewModel.cs

@@ -101,6 +101,7 @@ private async Task<List<OpenArtCustomNode>> ParseNodes(List<string> nodes)
 
         var installedNodesNames = new HashSet<string>();
         var nameToManifestNodes = new Dictionary<string, PackageExtension>();
+        var addedMissingNodes = new HashSet<string>();
 
         var packagePair = SelectedPackagePair;
 
@@ -160,9 +161,11 @@ out var manifestExtension
                     IsInstalled = installedNodesNames.Contains(node),
                 };
 
-                // Add missing nodes to the list
+                // Add missing nodes to the list (deduplicate by title)
                 if (
-                    !currentSection.IsInstalled && nameToManifestNodes.TryGetValue(node, out var manifestNode)
+                    !currentSection.IsInstalled
+                    && nameToManifestNodes.TryGetValue(node, out var manifestNode)
+                    && addedMissingNodes.Add(manifestNode.Title)
                 )
                 {
                     MissingNodes.Add(manifestNode);