When trying to modify villager inventory with /data (specifically the item Count) it always either adds or duplicates the value.

How to recreate:

1. /summon villager
2. Drop some items that villager can pick up (in this case 3 potatoes)
3. /data get entity @e[type=villager,sort=nearest,limit=1] Inventory
   The item Count will show 3
4. /data modify entity @e[type=villager,sort=nearest,limit=1] Inventory[0].Count set value 1
   /data get entity @e[type=villager,sort=nearest,limit=1] Inventory
   The Count now will be 4
5. /data modify entity @e[type=villager,sort=nearest,limit=1] Inventory insert 0 value {}
   /data get entity @e[type=villager,sort=nearest,limit=1] Inventory
   Item Count is now doubled and shows 8
6. /data modify entity @e[type=villager,sort=nearest,limit=1] Inventory append value {id:"minecraft:wheat",Count:1b}
   /data get entity @e[type=villager,sort=nearest,limit=1] Inventory
   The potato Count is now doubled again and shows 16
   if this step is repeated the values will double again, showing 32 potatoes in slot 0 and 3 wheat in slot 1, one more and counts will be 64 and 7