[bug report] cascade selection box when the drop-down options are repeated, the option will be cleared after selection, and the drop-down box will expand again to display the last selected path

### Element UI version
2.15.0

### OS/Browsers version
windows10/goole

### Vue version
2.6.12

### Reproduction Link
https://github.com/ElemeFE/element/blob/dev/FAQ.md#%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98

### Steps to reproduce
API网站Cascader 级联选择框当选项存在重复时，选中后清空选项，再次展开下拉框显示路径为重复选项第一次存在的位置

### What is Expected?
清空选项后，同步选择路径

### What is actually happening?
清空选项后，再次下拉，显示路径错误