issue 1253 - prevent multiple logins with same username

Resolves #1253 
Instead of an `if` spanning the entire function length, consider using the opposite condition and returning:
```cpp
// If the multilogin is allowed, skip this check entirely:
if (/* IsAllowMultiLogin */)
{
  return true;
}

// The rest of the code
```
This makes the code less deep, which helps readability quite a lot, especially with the comments saying what the `if` is doing.