`#:project` directive duplicate not detected

```sh
$ dotnet --version
10.0.100-rc.1.25451.107
```

Repro:
1. Create a project under directory `./MyProject/`
2. Create an `app.cs` in directory `.` containing the following:
```cs
#:project ./MyProject
#:project ./MyProject/MyProject.csproj
```

Expected: a duplicate project directive error is reported
Actual: no error

@jjonescz