Hello,
It is possible to receive a duplicate of a blue print piece you already know, meaning the trade does not necessarily give you a new blue print. For example if this is your current blue print count:
203
422
235
If you select the last blue print piece and trade in 2 copies of it, the number of copies will drop from 5 to 3. However, this trade in could give you a copy for any of the other 7 pieces you already own and not for the blue print you're missing (marked with a "0").
For further investigation as to what precisely happened in your case, please
contact support with a ticket. Thanks!