replaceitem for specific player replaces item for all players.

Using a command block set up like.

clear @[name=USER] 0 0 arrow

into inverted redstone to trigger the next command block if the first one fails

replaceitem @a[name=USER]  entity @a[name=USER] slot.wapon.offhand 0 arrow 1 0

what this does is replace every user on the realms item, not just the targeted player.