We won’t be able to equip the same item in more than one set unless we have more than one of the same item.

That’s to say if you want to equip two Spinel Ring’s to your Goldsmith set, and also equip two Spinel Ring’s to your Weaver set, you will need a total of four Spinel Ring’s.
How do you know this is the case? I don't see that stated anywhere.