解决方案
我不知道这是否是***的，但它适用于我，我认为这是有道理的。 EF创建一个重复的相关性，因为它们是分离的，
我以这种方式改变了我的代码：

SOLUTION I don't know if it's the best, but it works for me and I think it has sense. EF creates a duplicate of the relatedentities because they were detached, I changed my code in this way:

public void AddOrUpdate(MyEntity entity) {
    var dbEntity = _context.MyEntities
      .Include(e => e.RelatedEntitis)
      .Where(e => e.ID == entity.ID)
      .SingleOrDefault();

    //replace entities with those taken from the context
    var relatedEntities = _context.RelatedEntities;
    var detachedEntities = entity.RelatedEntities;
    entity.RelatedEntities = new List<RelatedEntity>();
    foreach (var ent in detachedEntities)
    {
      entity.RelatedEntities.Add(relatedEntities .Where(e => e.ID == hop.ID).SingleOrDefault());
    }

    var newRelated = entity.RelatedEntitis.ToList<RelatedEntity>();
    var dbRelated = dbEntity.RelatedEntity.ToList<RelatedEntity>();

    _context.Entry(dbEntity).CurrentValues.SetValues(entity);
    _context.Entry(dbEntity.RelatedEntity).CurrentValues.SetValues(entity.RelatedEntitis);

    var comparer = new EqualityComparer();
    var addedRelated = newRelated.Except(dbRelated, comparer).ToList<RelatedEntity>();
    var deletedRelated = dbRelated.Except(newRelated, comparer).ToList<RelatedEntity>();

    addedRelated.ForEach(e => dbEntity.RelatedEntity.Add(e));
    deletedRelated.ForEach(e => dbEntity.RelatedEntity.Remove(e));
}