Trend Micro:

A survey of the top 50 free apps available for download in Google Play revealed that almost 80% of the samples had fake versions. These apps span a wide range of categories in Google Play, including Business, Media & Video, and Games.

Of roughly the top 10 apps in each category in Google Play, fake versions of the following were available:
• 100% of the apps categorized under Widgets, Media & Video, and Finance
• 90% of the apps categorized under Business, Music & Audio, and Weather
• Approximately 70% of the apps categorized under Games, Books and Reference, and Live Wallpapers

Typically, a fake version of an app is created by copying the .apk file and inserting code, with the possible goal of creating malware. While there are certainly knock-offs that have made their way onto Apple’s app store, there’s a review process to prevent this sort of malware factory.