Small suggestion would be to replace the array with an object structure in that sense you don't need to check if the item exists as you can set it regardless like:

ids[ attachment[1] ] = true;

Then you can just return the keys from it, if you only need the values:

return Object.keys( ids );

Will the file name always match?

The thought behind the question: Someone uploads the image hamilton-the-musical.jpg, this will generate fallback image hamilton-the-musical.webp. Later the image hamilton-the-musical.jpeg is uploaded (note the e), this will generate images of a different name for the fallback -- probably hamilton-the-musical-2.webp.

🔢 The same question applies for srcset.

This is actually a good point but I think it is out of scope for this issue. The problem is bigger that just properly replacing webp files here. I tried to replicate your edge case and it looks like in this case webp images for the second image will override webp images for the first image 🤔.

I uploaded 20180607_144302.jpg image first, webp images have been created correctly. Then I uploaded the same image but with another extension 20180607_144302.jpeg and in this time webp images overriden webp images generated for the first image. Here is what I got on the server:

-rw-r--r-- 1 www-data www-data 161891 Jul  8 18:15 20180607_144302-1024x576.jpeg
-rw-r--r-- 1 www-data www-data 161891 Jul  8 18:13 20180607_144302-1024x576.jpg
-rw-r--r-- 1 www-data www-data 170276 Jul  8 18:14 20180607_144302-1024x576.webp
-rw-r--r-- 1 www-data www-data   8220 Jul  8 18:15 20180607_144302-150x150.jpeg
-rw-r--r-- 1 www-data www-data   8220 Jul  8 18:13 20180607_144302-150x150.jpg
-rw-r--r-- 1 www-data www-data   8724 Jul  8 18:14 20180607_144302-150x150.webp
-rw-r--r-- 1 www-data www-data 330858 Jul  8 18:15 20180607_144302-1536x864.jpeg
-rw-r--r-- 1 www-data www-data 330858 Jul  8 18:14 20180607_144302-1536x864.jpg
-rw-r--r-- 1 www-data www-data 331106 Jul  8 18:14 20180607_144302-1536x864.webp
-rw-r--r-- 1 www-data www-data 529186 Jul  8 18:15 20180607_144302-2048x1152.jpeg
-rw-r--r-- 1 www-data www-data 529186 Jul  8 18:14 20180607_144302-2048x1152.jpg
-rw-r--r-- 1 www-data www-data 506556 Jul  8 18:14 20180607_144302-2048x1152.webp
-rw-r--r-- 1 www-data www-data  17533 Jul  8 18:15 20180607_144302-300x169.jpeg
-rw-r--r-- 1 www-data www-data  17533 Jul  8 18:13 20180607_144302-300x169.jpg
-rw-r--r-- 1 www-data www-data  18692 Jul  8 18:14 20180607_144302-300x169.webp
-rw-r--r-- 1 www-data www-data  96605 Jul  8 18:15 20180607_144302-768x432.jpeg
-rw-r--r-- 1 www-data www-data  96605 Jul  8 18:14 20180607_144302-768x432.jpg
-rw-r--r-- 1 www-data www-data 102924 Jul  8 18:14 20180607_144302-768x432.webp
-rw-r--r-- 1 www-data www-data 934789 Jul  8 18:15 20180607_144302.jpeg
-rw-r--r-- 1 www-data www-data 934789 Jul  8 18:13 20180607_144302.jpg
-rw-r--r-- 1 www-data www-data 826980 Jul  8 18:14 20180607_144302.webp

This doesn't look right although this is a very unlikely edge case. I think it is worth creating a separate issue for it 🤔.

cc @felixarntz @adamsilverstein

This was previously discussed and should have been fixed in the plugin - see #358 - not sure why you still see this happening. I'm not sure this fix is great though, it skips the duplicate generation; instead behavior should be like or use wp_unique_filename.

For core I am working on including this in the core patch as well, I tried one approach in 93542af (#2393) - that isn't quite finished - note there can actually be three variations - cat.jpe, cat.jpeg, and cat.jpg - yes ".jpe" is also valid!

Another option would be to incorporate the fix into WP_Image_Editor::generate_filename or save - again the tricky part is making sure overwriting works when you want it to (ie. regenerate images).

This is actually a good point but I think it is out of scope for this issue. The problem is bigger that just properly replacing webp files here.

Thanks for testing this. I'm happy if y'all decide it is out of scope for this ticket. It seems like a much bigger problem than I thought when asking the question :)

The problem of avoiding the override was indeed fixed in #358 - however that shouldn't have affected the PR here anyway, since realistically the problem is only on the JPEG (jpeg/jpg/jpe) side. Now that #358 is in place, no conflicting WebP image would be created so we should be good for now.

With that said, I agree this is a valid point, and I think a much more stable implementation for the replacement would be to replace the full file name for the image/webp version per size with its corresponding full file name for the image/jpeg version. Let's open a follow-up issue to add that.

The WordPress test suite includes a helper for doing this, see source code.

🔢 Applies to other tests in this file too.

The problem of avoiding the override was indeed fixed in #358 - however that shouldn't have affected the PR here anyway, since realistically the problem is only on the JPEG (jpeg/jpg/jpe) side. Now that #358 is in place, no conflicting WebP image would be created so we should be good for now.

With that said, I agree this is a valid point, and I think a much more stable implementation for the replacement would be to replace the full file name for the image/webp version per size with its corresponding full file name for the image/jpeg version. Let's open a follow-up issue to add that.