Customizability is a key feature of the Android operating system that differentiates it from Apple's iOS. One concrete feature that gaining popularity is called "app virtualization''. This feature allows multiple copies of the same app to be installed and opened simultaneously (e.g., with multiple accounts logged in). Virtualization frameworks are used by more than 100 million users worldwide. As with any new system features, we are interested in two aspects: (1) whether the feature itself introduces security risks and (2) whether the feature is abused for unintended purposes. This paper conducts a systematic study on the two aspects of the app virtualization techniques. With a thorough study of 32 popular virtualization frameworks from Google Play, we identify seven areas of potential attack vectors and find that most of the frameworks are susceptible to them. By deeply investigating their ecosystem, we show, with demonstrations, that attackers can easily distribute malware that takes advantage of these attack vectors. In addition, we show that the same virtualization techniques are also abused by malware as an alternative and easy-to-use repackaging mechanism. To this end, we design and implement a new app repackage detector. After scanning 250,145 apps from app markets, it finds 164 repackaged apps that attempt to steal user credentials and private data.