[Bug Report] custom theme, CSS repeated multiple times

### Element UI version
2.4.2

### OS/Browsers version
ubuntu16.4

### Vue version
2.5.17-beta.0

### Reproduction Link
[https://jsfiddle.net/mmx38qxw/localproject](https://jsfiddle.net/mmx38qxw/localproject)

### Steps to reproduce
按照官网2种自定义主题步骤操作,设置好新的主题后,打开项目页面,从控制台可以看到button,input等空间的样式重复多次,有4次,7次甚至更多次.
使用webpack css去重插件也不生效

### What is Expected?
css样式不重复

### What is actually happening?
css样式重复