How do I fix 1000+ multiplied photos issue on iPhone?

Boy, that's weird! Is the original in an album? Duplicates don't have albums (well, normally) so you could pick out the duplicates on your Mac with a Smart Album using Album is not Any, maybe with a file name, and dump them all at once.

Honestly, I can't think of anything that could do this. A funny thing is that I just did a "duplicate" command on my phone and, since like you I use Optimize, it had to download the image first, and it took a few seconds. Even when pre-dowloaded, it takes a second, so 1000 pictures? This couldn't happen through any normal commands.

Do you have a cat?

I'm not sure what you mean by multiplied -- duplicated? Are you saying that each picture has over 1000 duplicates? Or 1000 pictures have been duplicated? I haven't seen anything like this from anyone else here.

It's hard to deal with this when you didn't tell us what device you're using. Is this happening on a Mac, an iPhone, an iPad, or what? What OS is the device using? Are you using iCloud Photos? Are you using Optimized Storage? What were you doing when this happened. We aren't there with you and can't see your phone, so it's hard to help with no information.

Let us know some details…