[backport core/1.40] fix: subgraph unpacking creates extra link to seed widget (#9572)

Backport of #9046 to `core/1.40`

Automatically created by backport workflow.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9572-backport-core-1-40-fix-subgraph-unpacking-creates-extra-link-to-seed-widget-31d6d73d365081f4b64ccdaffa508e8e)
by [Unito](https://www.unito.io)

Co-authored-by: Christian Byrne <cbyrne@comfy.org>

browser_tests/tests/subgraph.spec.ts

@@ -375,6 +375,45 @@ test.describe('Subgraph Operations', { tag: ['@slow', '@subgraph'] }, () => {
     })
   })
 
+  test.describe('Subgraph Unpacking', () => {
+    test('Unpacking subgraph with duplicate links does not create extra links', async ({
+      comfyPage
+    }) => {
+      await comfyPage.workflow.loadWorkflow(
+        'subgraphs/subgraph-duplicate-links'
+      )
+
+      const result = await comfyPage.page.evaluate(() => {
+        const graph = window.app!.graph!
+        const subgraphNode = graph.nodes.find((n) => n.isSubgraphNode())
+        if (!subgraphNode || !subgraphNode.isSubgraphNode()) {
+          return { error: 'No subgraph node found' }
+        }
+
+        graph.unpackSubgraph(subgraphNode)
+
+        const linkCount = graph.links.size
+        const nodes = graph.nodes
+        const ksampler = nodes.find((n) => n.type === 'KSampler')
+        if (!ksampler) return { error: 'No KSampler found after unpack' }
+
+        const linkedInputCount = ksampler.inputs.filter(
+          (i) => i.link != null
+        ).length
+
+        return { linkCount, linkedInputCount, nodeCount: nodes.length }
+      })
+
+      expect(result).not.toHaveProperty('error')
+      // Should have exactly 1 link (EmptyLatentImage→KSampler)
+      // not 4 (with 3 duplicates). The KSampler→output link is dropped
+      // because the subgraph output has no downstream connection.
+      expect(result.linkCount).toBe(1)
+      // KSampler should have exactly 1 linked input (latent_image)
+      expect(result.linkedInputCount).toBe(1)
+    })
+  })
+
   test.describe('Subgraph Creation and Deletion', () => {
     test('Can create subgraph from selected nodes', async ({ comfyPage }) => {
       await comfyPage.workflow.loadWorkflow('default')