Invalid runtimeconfig.json is produced when duplicate FrameworkReference exists in 3.0preview

[yyjdelete]

Move from dotnet/aspnetcore#4490

Step

dotnet sdk 3.0.0-preview-27122-01 is needed.

1. dotnet new webapi
2. change the csproj to add an FrameworkReference <FrameworkReference Include="Microsoft.AspNetCore.App" />
3. dotnet run

It can also be reproduce with dotnet new console by add more than one same FrameworkReference

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>
</Project>

NOTE:
When the number of duplicated FrameworkReference is changed, the xxx.runtimeconfig.json only change after an dotnet clean and build, seems the cache and build have different rule in this special case? Not sure if it's another issue, or just an part of this.

Expected

Build error, or run successful.

{
  "runtimeOptions": {
    "tfm": "netcoreapp3.0",
    "framework": {
      "name": "Microsoft.AspNetCore.App",
      "version": "3.0.0-preview-18579-0056"
    }
  }
}

Actual

Build successful, but run failed with Invalid runtimeconfig.json. See the duplicate Microsoft.AspNetCore.App in frameworks here.

{
  "runtimeOptions": {
    "tfm": "netcoreapp3.0",
    "frameworks": [
      {
        "name": "Microsoft.AspNetCore.App",
        "version": "3.0.0-preview-18579-0056"
      },
      {
        "name": "Microsoft.AspNetCore.App",
        "version": "3.0.0-preview-18579-0056"
      }
    ]
  }
}

Details

dotnet --info
.NET Core SDK（反映任何 global.json）:
Version: 3.0.100-preview-009812
Commit: e3abf6e

运行时环境:
OS Name: Windows
OS Version: 10.0.18290
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.0.100-preview-009812\

Host (useful for support):
Version: 3.0.0-preview-27122-01
Commit: 00c5c8bc40

.NET Core SDKs installed:
2.1.401 [C:\Program Files\dotnet\sdk]
2.1.500-preview-009404 [C:\Program Files\dotnet\sdk]
2.1.500 [C:\Program Files\dotnet\sdk]
2.1.600-preview-009426 [C:\Program Files\dotnet\sdk]
2.2.100-preview3-009430 [C:\Program Files\dotnet\sdk]
2.2.100 [C:\Program Files\dotnet\sdk]
3.0.100-preview-009812 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.0-preview3-35497 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.0-preview3-35497 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview-18579-0056 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.0-preview3-27014-02 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview-27122-01 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0-alpha-27128-4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

[dsplaisted]

This should be fixed