This is a classic race condition, "check then do". Consider what happens if two database connections try to do the same thing at roughly the same time.
| Connection A | Connection B |
|---|---|
| if not exists... | |
| if not exists... | |
| true | |
| true | |
| insert... | |
| insert... | |
| success | |
| duplicate |
To avoid this, "do then check". Do the insert and catch the duplicate key error.