InvalidOperationException on Attaching POCO entity to context

I'm experiencing a trouble with attaching a POCO entity to its context. I'm getting an InvalidOperationException with the following message:

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

Actually, I only got this exception with a single item. I did not experience this kind of exception when I attached different items aside from this specific item.

I'm using the following code:

 ItemDocument itemDocument = new ItemDocument();
 Item item = GetItem(itemID); // the item I got here is detached
 using (SampleContext context = new SampleContext()){
      context.Items.Attach(item);  // I got the exception here
      context.LoadProperty(item, "Classifications");
      itemDocument.Classification = item.Classifications.Select(c => c.Name).ToList开发者_开发百科();
 }

Thanks.

What about trying to Detach the old object from context before attaching new one with the same key?