Oh God, the nerd boners: Fred Armisen will be co-hosting this week’s “This American Life” in character as Fake Ira Glass. The episode’s theme is doppelgängers, so in addition to being a dangerous vortex of NPR self-awareness and square glasses, it’s also thematically appropriate. According to the “TAL” blog, Armisen developed the impression for Saturday Night Live, but Glass was deemed not famous enough for ridicule, which is probably fair. (As we look forward to this no doubt wonderful and deeply affectionate doppelgänger-ing, let us take a moment to revisit the greatest “TAL” send-up ever, Kasper Hauser’s spoof episode from 2007.) Each week, we bring you stories on a theme. This week, gently making fun of Ira Glass, but from a place of sincere love and admiration.