Multiple instances of product in one order.

For an example, I have been using MS Northwind example. One problem I have
run into is adding the same item twice in an order. I am keeping track of
time in my database. For instance, Project1 might be from 8:00AM to 12PM,
then Project2 from 12PM to 3PM, then Project1 again from 3PM to 5PM.

If you try to add the same product in the sample Northwind database, it
comes up with the same error. I know the reason it is coming up is because it
has to have its own primary key and cannot duplicate. How should I go around
this so I can add the same "product" in the same order?

Thanks for any comments.