[Bug Report] Importing SCSS style results in duplicated styles

### Element UI version
2.15.6

### OS/Browsers version
macOS Big Sur 11.6

### Vue version
2.6.14

### Reproduction Link
https://codesandbox.io/s/patient-lake-9ohxy

### Steps to reproduce
1. Import Element SCSS following instructions in https://element.eleme.io/#/en-US/component/custom-theme#update-scss-variables-in-your-project
2. Inspect compiled CSS (printed on screen in reproduction link)

### What is Expected?
Each css rule appears only once

### What is actually happening?
Many CSS rules appear duplicated (some appear 11 times! See e.g. ".el-textarea {")

This probably happens because the smaller files are imported multiple times in different components. For example, "input.scss" is imported both in "index.scss" and "select.scss", leading to duplication