Double shulker shells does exactly what it sounds like, it doubles the shulker shells that you receive from killing a shulker.
Link: https://www.spigotmc.org/resources/doubleshulkershells.82365/
Github (also listed on Spigot): https://github.com/tgb20/DoubleShulkerShells
Why add this pack?
It would remove the need to use datapacks to get double shulker shells.