Here's why I plan to buy more shares of Casey's General Stores for my daughter, even after her investment has nearly doubled in less than two years.Buying more of a winning stock is one of the hardest things to do in investing. This notion is especially true when the stock's valuation has risen alongside its price.However, investors should avoid anchoring to the original purchase price and valuation of their winning investment. Saying we shouldn't add to a winning stock until it hits a "reasonable" valuatio ...