An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

You must execute the query before detach it.

//this gets error

public IEnumerable<USER_CONTACT> SelectAll(Guid userId)
        {
            //create query
            var query = from q in unitOfWork.UserContacts
                        select q;

            foreach (var item in query)
            {
                unitOfWork.UserContacts.Detach(item);
            }
            return query;
        }


//no problem
public IEnumerable<USER_CONTACT> SelectAll(Guid userId)
        {
            //create query
            var query = from q in unitOfWork.UserContacts
                        select q;

            var result = query.ToList();
            foreach (var item in result)
            {
                unitOfWork.UserContacts.Detach(item);
            }

            return result;
        }