ASP.NET Identity - Multiple object sets per type are not supported

You do have two DbSets` of the same type.

IdentityDbContext<T> itself contains Users property declared as:

public DbSet<T> Users { get; set; }

You're declaring second one in your class.

review this file "ApplicationDbContext.cs", remove the line, generated automatically by scaffold last, should be like this:

public System.Data.Entity.DbSet<Manager.Models.ApplicationUser> IdentityUsers { get; set; }

This issue can arise from using scaffolding to create a View. You probably did something like this: View > Add > New Scaffold Item... > MVC 5 View > [Model class: ApplicationUser].

The scaffolding wizard added a new line of code in your ApplicationDbContext class.

public System.Data.Entity.DbSet<RecommendationPlatform.Models.ApplicationUser> IdentityUsers { get; set; }

Now you have two DbSet properties of the same type which not only causes an exeptions to be thrown in the FindAsync() method but also when you try to use code-first migrations.

Be very careful when using scaffolding or even better don't use it.