So I have one last collectable to find so I have asked the Chief of my tribe to help point it out to me but he/she keeps pointing out to the ones I all ready have. Why does he/she keep doing this or have I misunderstood this feature?

The chief will point out a rare collectible, meaning any rare collectible. The collectibles you already have in your collections have no effect on which rare collectible will appear. As your collections become more complete, the likelihood of duplicates significantly increases.

Oh what a bummer:(

That can actually be pretty handy if you need tech points, since duplicates are worth 1500 each. Of course, late in the game (as you probably are), you'd have to like changing your villagers' outfits a lot to need many tech points.