Warn about duplicate Examples in Scenario Outlines

[idg10]

If a Scenario Outline contains the same line twice in its Examples, this is highly likely to be a mistake. (It certainly was when we did it...)

Up until fairly recently, if the duplicate was the last item in the list, this caused a crash when using the MSTest runner, but the error did not make it at all obvious what had gone wrong.

After a fair amount of investigation, we worked out that the feature cleanup would run when it hit an Example that was the same as the final Example, and then when it went on to run the next test, a NullReferenceException would occur inside the TestInitializeAsync method generated in the .feature.cs file because the teardown had already set testRunner to null.

We didn't initially realise that the root cause was the duplication of an Example. It took a long time debugging the innards of Reqnroll and MSTest before we understood what was happening.

We'd have understood the problem much earlier if Reqnroll had simply told us that our Examples contained duplicate entries.

Upgrading to Microsoft.NET.Test.Sdk 17.11.1 and MSTest 3.6.0 stops the crashes. But it also masks the problem—with the latest versions of these, there's no visible consequence of having duplicate entries. But since it seems likely to be a mistake to have duplicates, it would still be good if Reqnroll could detect this and raise a warning.

[gasparnagy]

That makes sense.
Basically warnings could be emitted during compilation (by the Reqnroll code generator) and/or by the IDE plugin (e.g. VS extension).

I don't think we have a "warning" infrastructure in any of these yet though. but probably doable.

If anyone wants to play with this, I would recommend trying it first in the VS extension.