Mountain View duplexes, assuming you pick the right area, continue to appreciate. If you're buying it with a friend or relative, be sure you both have the same goals, and decide up front what happens when one of you want to get out of the property. Most problems happen when one person wants to sell, and the other one doesn't.
A duplex is a great first step into the housing market. The next step would be to keep it as a rental, and then buy a single family home for you to live in. There are a couple of good duplexes on the market in Mountain View right now. Please contact me, and I can give you additional information.