fix(custom-element): optimize slot retrieval to avoid duplicates

packages/runtime-dom/src/apiCustomElement.ts

@@ -685,11 +685,23 @@ export class VueElement
     if (this._teleportTargets) {
       roots.push(...this._teleportTargets)
     }
-    return roots.reduce<HTMLSlotElement[]>((res, i) => {
-      res.push(...Array.from(i.querySelectorAll('slot')))
-      return res
-    }, [])
+
+    const seen = new Set<HTMLSlotElement>()
+    const slots: HTMLSlotElement[] = []
+    for (const root of roots) {
+      const found = root.querySelectorAll<HTMLSlotElement>('slot')
+      for (let i = 0; i < found.length; i++) {
+        const slot = found[i]
+        if (!seen.has(slot)) {
+          seen.add(slot)
+          slots.push(slot)
         }
+      }
+    }
+
+    return slots
+  }
+
   /**
    * @internal
    */