It's not the position of the staple and hanko that would tell them. I've got two landing permissions and two "departed" stamps, and the paper's stapled to the second-to-last page. I could say it was from a previous trip if there was no electronic record-keeping, but if there was, well, I didn't buy anything duty-free on the last trip, only this one.

If it's just a matter of showing the paper and the item to prove it's leaving Japan, that's a piece of cake, since I plan on regularly using the phone I bought even outside Japan(bought one that can be unlocked by docomo upon presentation of a gaijin card or Japanese driver's license and 3150 yen).

EDIT: For others who stumble upon this thread in the future, I just handed it to the people at the Customs desk at Haneda Airport (right after the Customs questioning/bag search) and they just took it, no questions asked. So all in all, pretty painless.