Maximiliano Sanchez de Bustamante
Because you can play cards in any order. It makes most sense (as in less rule breaking) that Seeing Double copies Trendsetting's ability and not the card that Trendsetting is mimicking. Which means Seeing doubles still could copy the same card Trendsetting is mimics or another one.
In other words you are copying the literal ability of the card and not whatever ability the card is currently the place holder for. This also removes the potential situation where you are doubling a double, because maybe you play another Seeing Double and copy the first seeing Double, which was copying Trend Setting which was copying a market card etc...
There may be an official response coming but the less game breaking rule is the card only copies the actual text on the card it's copying.
+1 I agree with Chris.
Also there is a paragraph in the rulebook about loop back to the original card: "... or the copying effect loops back to the original card, the chameleon card is considered to have its own set icon and value"