In Minecraft 1.13.2 If you take any filled bottle with experience less than 1408/1408 and put it in the second slot of "XP Hopper" It will dupe into the second slot.

To make the matter worse it can be sucked out with a hopper and pumped into a chest.

Thank's you have the 1.3 version of xpstorage?

Fix in version 1.6