is there anyway to upgrade latest version of xcode and also keeping the previous version? Like I'll upgrade my xcode from version 8.3 to version 9.0, how can I also keep the version 8.3 in my mac? will duplicate do the job done? I'm on mac-os sierra.
The Xcode copy that was downloaded from the AppStore is always managed by the AppStore app; even if you move the bundle to a different location (folder), the next time you download Xcode from the store it will be overwritten.
You should always keep the "managed" (purchased) version up to date, and download any necessary older versions from Apple's developer website (log in needed).
In your particular case:
/Applications/Xcode_old/Xcode.app (to avoid filename collision with Xcode 9, residing in Applications/Xcode.app)