Investors hearing that Dollar General's IPO will price Thursday might be forgiven for thinking, "Isn't that already public?" It was, not too long ago. The 70-year-old discount retail giant traded until July 2007, when equity firm KKR took it private. Now, like recent IPOs Vitamin Shoppe (VSI), Education Management (EDMC) and RailAmerica (RA), it's returning to the market after an absence of just a few years. These deja vu debuts are the product of