Exception "Scheme already exists" when trying to implement multiple authentication schemes

Question

For a Azure app service that uses two different app registrations for internal and external users I want to implement multiple authentication schemes. I use a ASP.NET Core web application (.NET6) with the following app settings:

"AzureAdInternal": {
    "Instance": "https://login.microsoftonline.com/",
    "Domain": "demo.example.com",
    "TenantId": "tenantId",
    "ClientId": "clientId",
    "ClientSecret": "clientSecret",
    "CallbackPath": "/signin-oidc"
  },
  "AzureAdExternal": {
    "Instance": "https://login.microsoftonline.com/",
    "Domain": "demo.example.com",
    "TenantId": "tenantId",
    "ClientId": "clientId",
    "ClientSecret": "clientSecret",
    "CallbackPath": "/signin-oidc"
	}

In the web app program.cs file two authentications for these sections are added:

// Internal
builder.Services
    .AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApp(
        builder.Configuration.GetSection(Configuration.AzureActiveDirectoryInternalIdentifier)
    )
    .EnableTokenAcquisitionToCallDownstreamApi(internalScopes)
    .AddMicrosoftGraph(builder.Configuration.GetSection(Configuration.MicrosoftGraphIdentifier))
    .AddInMemoryTokenCaches();

// External
builder.Services
    .AddAuthentication()
    .AddMicrosoftIdentityWebApp(
        builder.Configuration.GetSection(Configuration.AzureActiveDirectoryExternalIdentifier),
        "openid2",
        Configuration.AzureActiveDirectoryExternalCookieIdentifier, false, "Display Name"
    );
    //.EnableTokenAcquisitionToCallDownstreamApi(externalScopes)
    //.AddDownstreamWebApi(
    //    Configuration.Downstream2ApiIdentifier,
    //    builder.Configuration.GetSection(Configuration.Downstream2ApiIdentifier)
    //);

After the deployment on the Azure portal the app fails to start. According to the application event log a second scheme cannot be added:

"Unhandled exception. System.InvalidOperationException: Scheme already exists: AppServicesAuthentication".

Has anybody an idea why this usage of multiple schemes fails?