Should explicitly call out behavior with duplicates

It'd be nice for the docs to call out explicitly what happens in the scenario where a duplicate value exists in the list. Does it throw an exception or does it just use one?

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#### Document Details

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* ID: 55a90074-6a83-041f-48dd-5f38df808375
* Version Independent ID: 28f8f3f9-4ec3-e4ae-206f-25b070894cef
* Content: [Enumerable.ToHashSet Method (System.Linq)](https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tohashset?view=netframework-4.7.2)
* Content Source: [xml/System.Linq/Enumerable.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Linq/Enumerable.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**