Less than a decade after the dot-com bust taught Wall Street and Silicon Valley investors that what goes up does not keep going up forever, a growing number of entrepreneurs and a few venture capitalists are beginning to wonder if investments in tech start-ups are headed toward another big bust. The chief evidence, according to industry experts and analysts, is the way venture capitalists and established companies are clamoring to give money to young companies, including those with only a shred of an idea. They are piling into me-too start-ups that imitate popular Web companies that already received financing.