PNG, and the deflate compression it uses, is usually pretty good at exploting redundancy. For example, this 18.1kB image only grows to 19.1kB when duplicated and concatenated:
By Contrast, it almost doubles in size to 35.7kB when the copy is mirrored before concatenation
I can't see why this is the case. From my understanding, the first stage of PNG compression, the filtering, shouldn't be able to make use of that redundancy in either of those cases. The second stage on the other hand, the deflate compression, shouldn't have a problem since most if not all of the duplicated parts should fit within it's 32 KiB sliding window size. (for reference the encoder I used was Google's zopflipng)
Why then can't PNG take advantage of the redundancy of that mirrored part?