I hadn't intented to include VX originally but I couldn't see any way to write about Xander in this context without cheating. I'd forgotten about HyenaXander until rebcake mentioned him upthread, but that's a "bespelled" version rather than true doppleganger anyway.

I think even lead vampires like Angelus, Spike, Trick and Holden might qualify in this group

Excellent point! *ponders* There's a great deal of variation that I'm sure that no one in the writers' room predicted back when Jesse was turned in S1; he's more like VX, his insecurities discarded & dark ego impluses in control; Holden Webster seems more or less "himself", implying he was more confident & mature than Jesse (or Spike) before he was turned. I've been thinking since I wrote this about the notion that both Spike and Xander are performing masculinity on various levels, as are Angel and Riley. It's a reversal of what used to be generally taught or implied in this culture, that masculinity (Man=Human) is an essential, stable identity, and femininity is a costume, an unstable construct.

which is why I think he finally finds his niche in *physical* construction), and to build this identity he uses the materials at hand

YES. Ironically it's Xander, not Buffy, who becomes the Hand by S7, while Buffy tries to take over the speechifying role. Obviously Xander does better with window frames than Buffy does with morale; she can't be the Heart when her own is so closed-off. "Materials at hand" also ties in with what Buffy has always practiced: "in a Slayer's hand, anything is a weapon" (as when she decapitates a vamp with a cymbal in TH.) A key theme of the series is that over-identifying with any one role or task to the exclusion of any other stifles growth: not only do the Hand, Heart, Mind, Spirit ("we're off to see the Wizard, the wonderful Wizard of...") need one another on the literal level (fighting demons), but in order to be whole human beings. I think of Giles burrowing behind reason & logic to mask emotional pain; IMO by the end of the show Xander could teach Giles a few lessons in emotional maturity. I DON'T consider the comics canon, but it's no wonder Xander is at Buffy's side in S8 and Giles isn't.

I think the problem for Xander is that he really can't see himself.

I hadn't thought of Old Xander as a doppleganger but that absolutely fits here. And I love the irony of Caleb's line. A lot of fandom seems to take Caleb literally and complain "since when is Xander the Eye"? It's more complicated than that; Caleb is a villain with his own agenda, complete with Bad Guy Snappy Line. D'Hoffryn is closer to the truth: Xander "sees with the eyes of love", i.e he's "the Heart". That's NOT the same as saying his interpretions/reactions are always true or right. Running to save Anya in Selfless is the reversal of running to kill Spike in Entropy on one level, on another level it's the same thing: putting what he wants ahead of everything else. It's no accident that Selfless is the episode where Buffy finally mentions The Lie from Becoming, one of the key moments where Xander was acting out of "love" for Willow and Buffy, at his most selfless and self-serving all at once.

In her episode notes for Anne & Dead Man's Party, 2maggie2 & local_max assume that underneath the surface Xander feels somewhat guilty about The Lie; I'd have to rewatch those episodes, but my initial reaction was that he doesn't even see it as something to apologize for because he was convinced he did the right thing. I certainly don't think he meant to hurt Buffy, but his failure to acknowledge that he did is one of the less pleasant sides of his character. Prior to Selfless, to the growth that's come after Entropy, Seeing Red, Grave, having to really go outside of his preconceived notions and comfort zone, having to "step up to the plate", so to speak, I don't think he would have been ready to hear the truth from Buffy about that incident. He doesn't apologize but he doesn't deny it, either. He just runs to the frathouse and tries to make things right.