[source mapping] Having a duplicate source breaks msbuild sdks restore about home

[JonDouglas]

/cc @nkolev92 & @donnie-msft related to #13469

[nkolev92]

What would you have expected to happen @jeromelaban compared to what the error message tells you today. (we have ideas ourselves, but don't want to lead you towards those :D )

[jeromelaban]

In itself, the message is does not feel easily actionable. Knowing the cause, I'd intuitively say that it should fail/warn earlier because the source mapping got overridden.

Also, maybe displaying all the known sources would have told that none of the available ones were considered?

[nkolev92]

Today it says:

Unable to resolve 'MSTest.Sdk (= 3.3.1)' for '.NETStandard,Version=v0.0'. PackageSourceMapping is enabled, the following source(s) were not considered: Microsoft Visual Studio Offline Packages, NuGet official package source.

A more powerful message would be to say "no sources" were considered potentially, because the package is not mapped.
Another idea is to warn the duplicate sources as well.

[jeromelaban]

Both messages would help for sure.

Another problem I see here is that the user for unoplatform/uno#16758 (reply in thread) stumbled on this issue had a lot of mappings defined at the user level. It feels like an incorrect use of the source mapping feature, as it forces any new package use to be added to the the mapping list (with errors like #13469). This situation is what led to this particular issue.

[donnie-msft]

had a lot of mappings defined at the user level

I wonder if my rough proposal in #13173 to have NuGet tooling able to create configs that <clear/> (reset) the list of package sources would have prevented creating those at the user-level.
Not the complete solution here, but perhaps a piece of the puzzle.