[Bug Report] Notifications are stacking instead of listing in loop

### Element UI version
2.10.0

### OS/Browsers version
Windows 10

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/rcomk6Le/4/

### Steps to reproduce
Try to instance notifications in a loop.

### What is Expected?
It is expected to have a list of notifications, not a stack.

### What is actually happening?
The notifications are stacking when in loop.