IOException when installing NuGet-package via paket

### Description

When installing the NuGet-package using paket, an IOException is thrown:

```
Paket failed with
-> Error during extraction of D:\projects\...\packages\FluentAssertions.Analyzers\fluentassertions.analyzers.0.11.4.nupkg.

In rare cases a firewall might have blocked the download. Please look into the file and see if it contains text with further information.
-> IOException: Die Datei "D:\projects\...\packages\FluentAssertions.Analyzers\analyzers\dotnet\cs\FluentAssertions.Analyzers.dll" ist bereits vorhanden.
```

### Expected behavior:

The package should be installed like using any other package manager

### Actual behavior:

An IOException (see above) is thrown and the package cannot be installed.

### Versions

* Which version of Fluent Assertions Analyzers are you using? --> 0.11.4
* Which .NET runtime and version are you targeting? --> .NET Core 2.1

### Additional Information

When inspecting the pacakge I noticed that there are two files with the same name conatined in the same folder.

Inizia ispezionando il .nupkg di FluentAssertions.Analyzers 0.11.4 e i relativi contenuti in analyzers/dotnet/cs, concentrandoti sulle voci duplicate FluentAssertions.Analyzers.dll segnalate da Paket. Riproduci l’installazione con Paket su .NET Core 2.1 e verifica che il pacchetto venga installato senza l’IOException.

Scritto dal modello di indicizzazione a partire dal testo della issue.