Q:

Can't insert same Entity (with different related properties-entity) to another Entity

I have an Entity Product who has a collection of Suppliers (which is also an Entity) and i have a Contract Entity who has a collection of Products.  A contract can have multiple products, even 2 same products but with a different Supplier. 
When i try to add this second product to my contract (same product but different Supplier this time) EF seems to ignore it and doesn't add it to the Contract. Products collection.  No errors but it doens't add it.  How can i bypass this behavior or setup my model/logic in a way that i can perform this action? 
Code:
// THIS CONTEXT RESIDES IN A USING BLOCK
// THE updateContracts. Products ARE COMING FROM ANOTHER CONTEXT and we are receiving this entity as a parameter

// fetching the contract we are updating from database
Contract contractFromDB = ctx. Contracts. Include(s)(...)Where(p => p.ID == updateContract. ID).FirstOrDefault();

// list to populate the products we have added
List<Product> productsToAddToDBContract = new List<Product>();

foreach (Entity. Product product in updateContract. Products)
{
  if (! contractFromDB.Products. Any(prod => prod. ProductName == product. ProductName))
  {
  // tried detaching it but didn't work
  //ctx. Entry(product).State = EntityState. Detached;
  productsToAddToDBContract. Add(product);
  }
}

foreach (Product product in productsToAddToDBContract)
{
// get these from DB and add those to DB contract products
Product productToAdd = ctx. Products. Include(p=> p.Suppliers).Where(prod=> prod. ProductName == product. ProductName).FirstOrDefault();
  if (productToAdd ! = null)
  {
   // HERE IS WHERE EF DOESN'T ADD THE SECOND PRODUCT (WHICH IS THE SAME BUT FROM ANOTHER SUPPLIER)
   contractFromDB.Products. Add(productToAdd);
  }
}

Thank you very much. 
EDIT:
I tried to remove the line where i fetched my product from Database 
Product productToAdd = ctx. Products. Include(...).Where(...).FirstOrDefault();

and just use the product i already fetched before and passing with the updateContract. Products, i got "The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects."  
so i went and attach the product from this iteration:
foreach (Product product in productsToAddToDBContract)
{
  // removed this line:
  // Entity. Product productToAdd = ctx. Products. Include(p=> p.Suppliers).Where(prod=> prod. ProductName == product. ProductName).FirstOrDefault();

  // and added this one
  ctx. Products. Attach(product);
  contractFromDB.Products. Add(product);
}

then i received '"Attaching an entity of type 'Supplier' failed because another entity of the same type already has the same primary key value.  This can happen when using the 'Attach' method or setting the state of an entity to 'Unchanged' or 'Modified' if any entities in the graph have conflicting key values.  This may be because some entities are new and have not yet received database-generated key values.  In this case use the 'Add' method or the 'Added' entity state to track the graph and then set the state of non-new entities to 'Unchanged' or 'Modified' as appropriate."  Note that the product already has a supplier attached/included to it but from another context (same as the product's context). 
This has to regarding the relation of Product and Supplier i think, or maybe because i am using different contexts, i don't know but i will investigate further and post back. 
Feel free to contribute in any way of course :)

A:

The issue was my model structure, i can't elaborate on how it was but the related entities relation was not setup correctly, i have created a new entity that holds a ProductId-SupplierId and others, tweaking it till i got what i wanted.  The reason why i got these errors (stated above) was because i didn't attach all the related entities when performing actions, when i refered the proper related entities to my product, the errors went away... :)
Hope this helps someone and forgive me for the somewhat unclear solution, i am in the middle of this project and time is an issue, for know the company wants a working solution not a clean one so i didn't not everything down ...
Kind regards!