Check for duplicate endpoint names on startup

[captainsafia]

Fixes #35436

[captainsafia]

Does this count as a breaking change!? Should there be an AppContext switch to skip this check?

This is technically not a breaking change. The policy has always been that duplicate endpoint names are not allowed in an application. The subtlety is that in MVC, route names map to endpoint names and we already have logic that checks to see if duplicate route names exist. There's also pre-existing logic in MVC that checks for duplicate endpoint names and throws an IOE. That logic was being superseded in the route name check.

aspnetcore/src/Http/Routing/src/EndpointNameAddressScheme.cs

Line 92 in c23a0e6

throw new InvalidOperationException(builder.ToString());

So, TLDR, we are throwing this exception in a new place but it's not a breaking change in the sense that it is not introducing a new assumption.

[javiercn]

Looks great!

[captainsafia]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/aspnetcore/actions/runs/1231642715

[captainsafia]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).