EF how to Attach entity

I have an object Manager, which has property EmailAccount, type EmailAccount:

public class Manager 
{
    .....
    public virtual EmailAccount EmailAccount { get; set; }
}

public partial class EmailAccount : BaseEntity
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string DisplayName { get; set; }
    ....
    public virtual Manager Manager { get; set; }
}

and mapping between these classes:

public class ManagerMap : EntityTypeConfiguration<Manager>
{
    public ManagerMap()
    {
        this.ToTable("Manager");
        this.HasKey(dp => dp.Id);
        this.Property(p => p.FirstName).IsRequired().HasMaxLength(250);
        this.Property(p => p.LastName).IsRequired().HasMaxLength(250);
        this.Property(p => p.Nickname).HasMaxLength(250);

        this.HasOptional(c => c.EmailAccount)
            .WithRequired(a => a.Manager)
            .Map(m => m.MapKey("ManagerId"));
    }
}

then in code I get object Manager from context and update it

var manager = _managerService.GetManager(user.Id);
manager.FirstName = model.FirstName;
manager.LastName = model.LastName;
manager.Nickname = model.Nickname;
manager.EmailAccount = mapper.Map<EmailAccount>(model.EmailAccount);

So, as we can see, EmailAccount is not gotten from context but uses an external object, but with Id (PK) the same as in context!

Then, before save changes, I try to attach this object to context:

_db.EmailAccounts.Attach(manager.EmailAccount);

but I got an error:

Saving or accepting changes failed because more than one entity of type 'Infrastructure.Asset.Messages.EmailAccount' have the same primary key value. Ensure that explicitly set primary key values are unique.

ADDED

_managerService.GetManager(user.Id) calls the following method:

    public Manager GetManager(string aspNetUserId)
    {
        var manager = _db.Managers
            .Include(p => p.EmailAccount)
            .Where(p => p.AspNetUser.Id == aspNetUserId)
            .FirstOrDefault();
        return manager;
    }

Why so, and how do I do it?