It comes in Trouble with the the plugin Deadchest and if you die you will get two backpacks
(I need the Entity name for the backpack)

Please try if the ImproveDeathChestCompatibility config option solves the issue for you.

thx but what can i do to delet the other backpack ?

The duplicates should get removed automatically after reconnecting, switching world or re-spawning.

Sry for this.

You can use the minecraft command /replaceitem

See here:
de
https://minecraft-de.gamepedia.com/Befehl/replaceitem
en
https://minecraft.gamepedia.com/Commands/replaceitem