Commented above regarding my testing results

I was testing more on this, and I noticed that neither Fixed nor Repeated options work with duoTone filters. I have no idea if there is a way to make them work, honestly. But we should do some research on that and then perhaps follow up with someone to see what to do about it. We can either:

1. Not implement fixed/repeated options
2. Find a workaround that's not too performance intensive
3. Add a notice after the settings about duotone not working
4. Disabling duotone entirely from the toolbar when either of those options are active

Now that you mention it kind of makes sense, I followed the same behavior that the Cover block has when selecting Fixed image, not sure what's the correct one. 🤔

Is there an easy way to try out stretching the image only until it covers the container? Otherwise, I'll leave the decision on this up to you. While testing several cases, this seemed really weird behavior for me, but I'm open to your opinion.

Duotone is fixed! Nice job! I'm approving, as I don't want to be an absolute pain on this issue, and everything works correctly.

However, the background issue is still bothering me a bit. Your link was an excellent find to explain what was happening.

I played around a bit, and I don't think it's worth adding JS to fix this issue as we can have a big impact on the performance of the page (we don't know how many of these blocks are people going to add in one page after all). What do you think?

While playing around I thought of a solution that could be viable, if you want to experiment a bit.

So, the issue in a nutshell is that when you tell a background to be fixed, its context will be calculated with the entire viewport: so the cover instruction essentially means to cover the entire viewport and not the wrapper container.

So, one way around it, would be to emulate the behavior of cover ourselves. So, instead of using background-size: cover, we calculate the dimensions of the container and hardcode that in the background-size (similarly to what we do with background-position).

Yes, this is a JS solution, but it's not a very expensive one, as we only need to do it as the user is editing the block, and then the front end should be able to render everything correctly via the PHP, causing no performance issues.

Oh, and last thing, again optional as I don't want to block this PR any longer: when choosing Fixed background option, the tag is transformed to a div, making the alt attribute useless. When I check Repeated background the alt option disappears from the sidebar, but it doesn't when I used the fixed one.

Both of these comments are optional, feel free to merge it if you are happy with the current state. None of it is a big deal.