Invalidoperationexception: there is already an open datareader associated with this command which must be closed first. With ASP.NET identity

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C#

ASP.NET

I'm trying to return data from database:

public List<DataModel> GET()
    {
        foreach (var users in db.Users) {
            var user = db.Users.Select(x => new DataModel()
            {
                Email = users.Email,
                UserName = users.UserName,
                FirstName = users.FirstName,
                LastName = users.LastName,
                Age = users.Age,
                Phone = users.Phone,
                Department = users.Department
            });
            return user.ToList();

        }

Here is db class:

public class ApplicationDbContext : IdentityDbContext<ApplicationUser, CustomRole,
int, CustomUserLogin, CustomUserRole, CustomUserClaim>
{
    public ApplicationDbContext()
        : base("DefaultConnection")
    {
    }

    public static ApplicationDbContext Create()
    {
        return new ApplicationDbContext();
    }
}

Here is the error:

InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.

Could you please tell what's wrong with this code?

What I have tried:

I have four record in database and the following code return four record but with first record data !!

public List<DataModel> GET()
           {


                   var user = db.Users.Select(x => new DataModel()
                   {
                       Email = db.Users.FirstOrDefault().Email,
                       UserName = db.Users.FirstOrDefault().UserName,
                       FirstName = db.Users.FirstOrDefault().FirstName,
                       LastName = db.Users.FirstOrDefault().LastName,
                       Age = db.Users.FirstOrDefault().Age,
                       Phone = db.Users.FirstOrDefault().Phone,
                       Department = db.Users.FirstOrDefault().Department
                   });
                   return user.ToList();

           }