android java, most recent modify date for a file

our android app auto backs up photos from mobile to internet. only want to backup the same photo once. our current bug results in backing up same photo from mobile phone to internet several times. our logic for determining unique photo is: [login to view URL](); [login to view URL](); if they are same as during previous scan / session, the photo does not get backed up. the part we cannot understand is user has not changed (deleted or edited) photos on their mobile device, but the [login to view URL] changes. once [login to view URL] changes, our program backs up the photo (but as the exact photo already backed up, we are backing up same photo multiple times). your job is to be smarter than us. either we are not using the [login to view URL](); command properly, or we should use a different logic. eg, is there another way to identify which photos are same as before? .. cannot use file size because different photos can have same size .. cannot use file name because some phones use identical standard names (eg., imag001, imag002, program backs up, then user deletes photos, takes new photos, device auto names imag001, imag002) thanks! tmr