Found proof for this completely by accident. Lovely.

Every one in existence is duped, so this is fine.

SALES:

http://www.tf2outpost.com/trade/21157593 Sold for Duped Moon Phantom (Outdated at 10, mini at 8) + Stormy 13th Billycock (3.5)

11.5 * 0.9 = 10.35 (rounding to 10.5)

Histories;

http://www.tf2outpost.com/item/440,2768758530,30121,5

http://www.tf2outpost.com/item/440,2770720441,337,5

http://www.tf2outpost.com/item/440,2744940014,108,5

http://www.tf2outpost.com/trade/22137201 Offered on Scorching Maddendoktor (9.8)

No overpay, =9.8 (rounding to 10)

Histories;

http://www.tf2outpost.com/item/440,2896335687,30121,5

http://www.tf2outpost.com/item/440,2896335697,30121,5

Moon Phantom mini:

http://www.tf2outpost.com/trade/20489714 Sold for Heart Chapeau (8.8)

8.8 * 0.9 = 7.92 (rounding to 8)

Histories:

http://www.tf2outpost.com/item/440,2770720441,337,5

http://www.tf2outpost.com/item/440,2698448605,397,5

I think this might be duped more than once, the history's super-confusing.

Using 8.

Let it ride.