Entity Framework Attach error: An object with the same key already exists in the ObjectStateManager - entity-framework

Question

I'm a newbie with Entity Framework, and I have looked at the questions with the same title, but I haven't found a satisfying answer yet.
Here is my class:
public class MyUser
{
public string FirstName { get; set; }
public virtual ICollection<ProfileSkillEdu> Skills { get; set; }
}
And in the controller I have:
[HttpPost]
public ActionResult EditProfile(MyUser user, string emailAddress)
{
try
{
if (ModelState.IsValid)
{
_unitOfWork.GetMyUserRepository().Update(user);
_unitOfWork.Save();
return View(user);
}
}
catch (DataException)
{
//Log the error
ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists see your system administrator.");
}
return View(user);
}
In my user repository:
public virtual void Update(TEntity entityToUpdate)
{
dbSet.Attach(entityToUpdate);
context.Entry(entityToUpdate).State = EntityState.Modified;
}
I keep getting An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key. at dbSet.Attach(entityToUpdate). I watched the variables and I found that if MyUser has only 1 Skill object, it's fine because when it does Attach, the key is unique (value is 0). But if MyUser has 2 skill objects, both primary key have the value of 0 and therefore the error.
Can someone explain why all the Skill object's primary key have the values of 0? Also, is there a simple solution to this problem? I thought it's supposed to be straight forward but I have been struggling with this for hours.
Edit:
I wonder if the problem is because of how I use the context.
In the controller definition I have:
public class MyAccountController : Controller
{
IUnitOfWork _unitOfWork;
public ActionResult EditProfile()
{
if (_user == null)
{
MembershipUser currentUser = Membership.GetUser();
if (currentUser == null)
{
return RedirectToAction("Logon", "Account");
}
Guid currentUserId = (Guid)currentUser.ProviderUserKey;
MyUserService svc = new MyUserService();
MyUser user = svc.GetUserLoaded(currentUserId); //this uses include/eager loading to get the Skills too.
if (user == null)
{
return RedirectToAction("Logon", "Account");
}
else
_user = user;
}
return View(_user);
}
}
In my UnitOfWork I have:
public class UnitOfWork:IUnitOfWork
{
private GlobalContext context = new GlobalContext();
private GenericRepository<MyUser> myUserRepository;
private GenericRepository<Skill> skillRepository;
.. and implementation of Save() and Dispose()
}