fix: properly mark slot rendered flag in production mode

fix vuejs#6997

Author: yyx990803

src/core/instance/render-helpers/render-slot.js

@@ -27,12 +27,14 @@ export function renderSlot (
   } else {
     const slotNodes = this.$slots[name]
     // warn duplicate slot usage
-    if (slotNodes && process.env.NODE_ENV !== 'production') {
-      slotNodes._rendered && warn(
-        `Duplicate presence of slot "${name}" found in the same render tree ` +
-        `- this will likely cause render errors.`,
-        this
-      )
+    if (slotNodes) {
+      if (process.env.NODE_ENV !== 'production' && slotNodes._rendered) {
+        warn(
+          `Duplicate presence of slot "${name}" found in the same render tree ` +
+          `- this will likely cause render errors.`,
+          this
+        )
+      }
       slotNodes._rendered = true
     }
     return slotNodes || fallback