Maybe, but remember that the first time we saw Athena she was impersonating Boomer and seducing Helo. It's not unprecedented for one Sharon to successfully impersonate another.
True enough, but at the time he thought he was left stranded on Caprica with a bunch of Toasters.. And at the time he was unaware of skin jobs, much less other "Sharon" models.
And of course later on, when he does find out, he shoots Athena :-)