[Bug Report] 2.15.12版本 打包报错 Duplicate identifier问题

### Element UI version
2.15.12

### OS/Browsers version
Chrome

### Vue version
2.7.14

### Reproduction Link
https://github.com/ElemeFE/element

### Steps to reproduce
1. npm install element-ui@2.15.12 安装2.15.12版本
2. npm run build 打包

### What is Expected?
打包成功

### What is actually happening?
打包出现错误：

ERROR in D:/frontend/node_modules/element-ui/types/element-ui.d.ts
688:14 Duplicate identifier 'Backtop'.
686 |
687 | /** Backtop Component */
> 688 | export class Backtop extends ElBacktop {}
| ^
689 |
690 | /** InfiniteScroll Directive */
691 | export const InfiniteScroll: PluginObject;

看了一下是element-ui.d.ts中出现对模块的重复声明问题