When you uninstall an app using usual pre-Lollipop approach (e.g. dragging to "trash" icon, or from App info > Uninstall), it's only uninstalled on current user. It can be confirmed by app showing "Not installed" on stock app manager. Android will detect that the app is still there, preventing the user to install the app with same package name.
It's due to Guest account (if it's activated), that when an app is installed, it's automatically installed on all users. So, you need to uninstall the app from there too, which can be troublesome in daily usage.
There are some approaches to uninstall the app completely (ordered by practicality):
- (from Owner account) Go to Settings > Apps, find and open the app info. Then, open the overflow menu (3 vertical dots), and choose Uninstall for all users.
- (from guest account) Deactivate the guest account if you don't need it (for more info: Removing the default guest user account from Android Lollipop)
- (from PC) Enter
adb uninstall <package> on command prompt/shell. This is the same as "uninstall for all users"
- (from guest/other accounts) Find the app and uninstall it normally.
- In case you can't uninstall it, try to remove the account and create a new one, then try to find and uninstall the app again.