fix(compiler-ssr) ensure that ssrHelpers are unique after merge from parent context (#3269)

fix #3268

packages/compiler-ssr/src/ssrCodegenTransform.ts

@@ -56,10 +56,11 @@ export function ssrCodegenTransform(ast: RootNode, options: CompilerOptions) {
 
   // Finalize helpers.
   // We need to separate helpers imported from 'vue' vs. '@vue/server-renderer'
-  ast.ssrHelpers = [
+  ast.ssrHelpers = Array.from(new Set([  
     ...ast.helpers.filter(h => h in ssrHelpers),
     ...context.helpers
-  ]
+  ]))
+  
   ast.helpers = ast.helpers.filter(h => !(h in ssrHelpers))
 }