I've purchased headphones in a foreign country and declared them when coming back. Do I have to re-declare them on subsequent trips?

A few months back I've purchased a set of expensive headphones in the US and dutifully declared them when coming back to Canada. Now I would like to travel to the US again and carry the same headphones in my backpack. When coming back to Canada, would I need to re-declare the item as technically it wasn't purchased in Canada? If not, how do I prove that the headphones have already been imported to Canada and I don't need to pay duty on them any longer?
If it matters, in the first trip I've verbally declared that I'm carrying the headphones when re-entering Canada by car and thus didn't have to fill out a customs declaration. I did not pay any duty as I was below the duty free limit at that time.
Best Answer
By the letter of policy, you shouldn't need to redeclare them when coming back in (the original declaration meant that you legally imported them the first time) but as per this memorandum you should properly complete a form BSF407 ("green form") "Identification of Articles for Temporary Exportation" as identification that you've identified the goods to the CBSA as having been legally in Canada previously. The form is apparently reusable.
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