Duplicate key

Hi, I just tried this library and with this file
```toml
abc="ABC"
[multi]
abc="ABC"
```
I get error `Internal\Toml\Exception\DuplicateKeyException: Duplicate key 'abc' at line 3, column 1`. This file
```toml
abc="Abc"
multi.abc="Abc"
```
is parsed correctly. If I understand the specification correctly, both should be valid. Can it be fixed?