I am building an application for recipe/meal planning, and i have come across a problem i cant seem to figure out.

i have a table for units of measure, where i keep the used units in, i only want unique units in here (for grocery list calculation and so forth)

but if i use a unit from the table on a recipe, the first time it is okay, nothing is inserted in units of measure, but the second time i get a "duplicate".

i suspect it has something to do with entitykey, because the primary key is identity column on the sql server (2008 r2)

for some reason it works to change the objectstate on some objects (courses, see code) and that does not generate a duplicate, but that does not work on the unit of measure

my insert methods looks like this :

public recipe Create(recipe recipe)
    {

        using (RecipeDataContext ctx = new RecipeDataContext())
        {
            foreach (recipe_ingredient rec_ing in recipe.recipe_ingredient)
            {
                if (rec_ing.ingredient.ingredient_id == 0)
                {
                    ingredient ing = (from _ing in ctx.ingredients
                                      where _ing.name == rec_ing.ingredient.name
                                      select _ing).FirstOrDefault();

                    if (ing != null)
                    {
                        rec_ing.ingredient_id = ing.ingredient_id;
                        rec_ing.ingredient = null;   
                    }
                }

                if (rec_ing.unit_of_measure.unit_of_measure_id == 0)
                {
                    unit_of_measure _uom = (from dbUom in ctx.unit_of_measure
                                            where dbUom.unit == rec_ing.unit_of_measure.unit
                                            select dbUom).FirstOrDefault();
                    if (_uom != null)
                    {
                        rec_ing.unit_of_measure_id = _uom.unit_of_measure_id;
                        rec_ing.unit_of_measure = null;
                    }
                }

                ctx.Recipes.AddObject(recipe);
//for some reason it works to change object state of this, and not generate a duplicate
                ctx.ObjectStateManager.ChangeObjectState(recipe.courses[0], EntityState.Unchanged);  
            }



            ctx.SaveChanges();
        }

        return recipe;
    }

My datamodel looks like this :

http://i.imgur.com/NMwZv.png