I am using this plugin called gravy: https://www.spigotmc.org/resources/gravy.45288/

when the players die, a new backpack appears in the inventory, while in the gravestone there is another one, thus they are doubled.

image: https://fotos.subefotos.com/49ccc5dc8d04c9bfb6493d3cff4234b5o.png

Sorry, I had it disabled, it works fine.
thanks for answering

Please turn on the ImproveDeathChestCompatibility config option.