As far as I know, dedupe only works on identical files or duplicates. Files in the same place with identical hashes can be handled automatically, while identical names with differing hashes usually requires some user input as the "correct answer" is not always obvious.
You could automate what you ask for, but it would have to be used with caution. It's not actually that unusual that an identical file exists in more than one place. Blindly removing the oldest of the two could make for example a program stop working when you download it again because some support file is missing (that some other program also used), and trying to figure out what happened could be annoying to say the least. For that reason it should not be a default behavior because it's not very "safe".
But specifically for photos and the like it would be ideal - I agree with that.
I don't think dedupe can currently do what you ask. You'd have to make a feature request (or upvote an existing one if you found that).
Meanwhile - it should be possible with some fairly basic scripting to output a list with hashes and dates from rclone, and then sort and delete with the external script. But then if you are willing to make that in the first place - why not just make a pull request and implement it directly into rclone? Anyone is free to make code suggestions
If you are not up to scripting anything then my last suggestion would be trying to find some photo-deduping software. I'm sure there exist several good free programs for this. This would have the benefit of being a ready-made solution that could work through a normal mount. The downside is it would be slower if you have a ton of pictures because it wouldn't have access to hashes via the mount. To be sure (and not just rely on size and date) it would have to actually download the file (for any size that is identical) and either do some analysis on the picture - or prompt the user. i think the latter is usually more common - but even though it requires some interaction it's at least pretty fast as you probably just get them side-by-side visually and click a button to decide. At least that's how I remember it working last time I used such software (sorry, can't remember a spesific name).