How to reproduce

Paste the following command in an always active repeating command block:

execute as @e[type=item] run data merge entity @s {Age:0s}

Now use the /give command to give yourself one of anything:

/give @p minecraft:stone 1

Expected behavior is that the player will be given one of that item. What really happens is that the player is given the item as specified in the /give command, but there is also plus one more dropped on the floor than what they asked for. In this case, the player was given two stone and not one.