This post shows you how to solve 'System.InvalidOperationException: Scheme already exists: Identity.Application' after identity scaffolding in ASP.NET Core.
For example
public class ApplicationUser : IdentityUser
{
public string FullName { get; set; }
}
Opening your Startup class, then modify your class as shown below.
services.AddIdentity<ApplicationUser, IdentityRole>()
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();