It's talking about goods that will be "consumed" (ie. used and/or left) in the destination country. Goods that are (in official terms) re-exported aren't dutible. You declare nothing that will be coming into and leaving the country with you.

How on earth would they know that something was purchased within 12 months unless you told them? Stop over thinking it. They don't know and you aren't silly enough to tell them and create pointless work for everyone. Yes?