I agree with Bark Flea; this feature is sorely needed across providers.

I suppose the checks would need to be carried out client side, since Proton does not have access to uploaded data.

I think this is a critical feature missing on all cloud services. Once the problem happens it is a nightmare to fix.

If you're (Proton Team) interested, I have a java program that can do this on my machine it is pretty light weight and might be able to work for this feature. It was published open on git forever ago but has since been upgraded and made proprietary. The old version I have isn't though ;)

Yes, especially with upload being so buggy .. it's easy to get many duplicates and very annoying to go through deleting manually