Diagnostics are shown multiple times if multiple documents belonging to the same project are added

[Vannevelj]

Title says it all.

I have it figured out and the culprit is

RoslynTester/RoslynTester/RoslynTester/Helpers/DiagnosticVerifier.cs

Line 325 in 6255f5c

foreach (var project in documents.Select(x => x.Project))

which does not take into account duplicated projects (e.g. documents belonging to the same project). I have confirmed this is the issue after I saw that the newest version of the helpers provided by Microsoft now use a HashSet<> prior to iterating. We can do the same or just add a Distinct() call after our Select().