Nyuuzyou's Minecraft-Skins-20M but deduped using BLAKE3 hashes (only catches if pixel values are exactly the same), then filtered to only valid 64x64 skins.
Format is a 2.6 GB ZIP archive containing 64x64 PNG skin files.
Tools used
PIL Image (Python), Google Colab (free CPU tier) and BLAKE3 (Python)