Why you may ask?  Because he kept sneezing.  Why did he do that?  It's never mentioned in the plot and we never see his Ganger sneeze.  Yet, at the same time, we're shown that Miranda, who has an inoperable blood clot, passes that to her Ganger.

There seems a double incongruity here.  Why give him a sneeze if they're not going to even mention it in the story?  Granted, Dicken didn't have much of a part, so perhaps sneezing was a shorthand way of giving him a character.  There's something missing here.

The second point has to do with "when did Amy get duplicated?"  My thought is that it happened around the time she was imprisoned in the Pandorica.  She was already seeing "eye-patch lady" before she was abducted by the Silence at the orphanage... and, she'd obviously already given birth, although that can be timey-wimey.

So, perhaps she was kidnapped slightly earlier... maybe at that botched cliffhanger at the end of Impossible Astronaut.  But, no, I think.  There's evidence that Amy was already either (A) Pregnant or (B) a Ganger when she first encounters the Silence in the bathroom, but the Silence seemingly compels her to both (A) Tell him that she's pregnant and (B) tell him what he must never know...  could that be in reference to her telling him he's going to die?  She seems awfully keen to do so throughout the series, with only Rory stopping her. Finally, in the Almost People she does tell him.

So, I think she's already been taken before the Impossible Astronaut, because...  her Ganger isn't pregnant!

If they'd impregnated her and then duplicated her, wouldn't the Ganger have the baby, too?  Just as Miranda's Ganger had the blot clot.

And what about the "morning sickness" episode that both she and River Song had? 

What does it all mean?

I'm sure she was abducted before this point in time.