After nearly quadrupling in value over the last two years, Sprouts Farmers Market offers investors market- beating potential, despite its recent dip. One of the hardest but most valuable lessons to learn with investing is to add to your winners. As counterintuitive as it may feel, winning stocks tend to keep winning over the long term. A perfect opportunity to put this lesson to work is with Sprouts Farmers Market (SFM -0.71%), a better-for-you grocery store chain. The company's share price has already quad ...