How to fix System.InvalidOperationException: Scheme already exists: Identity.Application

The error message System.InvalidOperationException: Scheme already exists: Identity.Application typically occurs when you're trying to add Identity to your ASP.NET Core project, but the scheme "Identity.Application" is already registered in the authentication options.

If you create an ApplicationUser class that extends the IdentityUser class, then add a FullName property

public class ApplicationUser : IdentityUser
{
    public string FullName { get; set; }
}

Here are a few steps you can take to resolve this issue:

Opening your Startup class, then modify your class as shown below.

// already exists: identity.application scheme
services.AddIdentity<ApplicationUser, IdentityRole>()
            .AddEntityFrameworkStores<ApplicationDbContext>()
            .AddDefaultTokenProviders();

Make sure you haven't accidentally configured Identity multiple times in your Startup.cs file. Check for duplicate calls to services.AddIdentity or any other related methods.

If you find duplicate configurations related to Identity in your Startup.cs, remove the redundant ones.