Using these commands inside command blocks to prevent a player from dropping items:

(1st Command Block Settings: Repeat, Unconditional, Needs Redstone)

/execute as @e[type=item] at @s on origin run data modify entity @e[type=item,sort=nearest,limit=1] Owner set from entity @s UUID

(2nd Command Block Settings: Chain, Conditional, Always Active)

/execute as @e[type=item] run data modify entity @s PickupDelay set value 0s

(3nd Command Block Settings: Chain, Conditional, Always Active)

/execute as @e[type=item] at @s on origin run tp @e[type=item,sort=nearest,limit=1] @s

If a player opens up their inventory while in creative mode and repeatedly drag and drop an item out of their inventory, after the first attempt unique per item, it will begin to double the amount of the item. 

While in survival mode if a player repeats the actions above, the item will not duplicate as is intended.