This is properly a silly question but if I use Complete Linux Installer (https://play.google.com/store/apps/details?id=com.zpwebsites.linuxonandroid), will it overwrite my current system? And if it does, is there a way to dual boot it? Hope you can help me. Thanks!
Looking at the description of the said app on the Google Play Store as referred by the OP:
Complete Linux Installer is an all in one solution to installing Linux distros on your Android device. Brought to you by the LinuxonAndroid project the app is designed to allow you to install a full Linux distro without touching your Android install.
So, it does not really "over-write" the Android installation. The emphasis in the above quote is mine.
It does pay to read the description on the Play Store thoroughly in order to avoid any confusions. If there is a confusion, the said developer would be delighted to receive an email asking for clarification as it shows that you as a customer is interested in the developer's app and might spur them on to improve it etc. :)