Having this command on a 1 tick cycle produces the expected result, except in the first tick when the game is loaded.

execute in overworld unless entity aec-0-0-0-1 positioned 0.0 0.0 0.0 run summon minecraft:area_effect_cloud ~ ~ ~ {Tags:["helper_entity"],Duration:-1,Age:-2147483648,WaitTime:-2147483648,UUID:[I;2796,0,0,1]}

An entity with the same UUID can't be summoned as it produces an error: Unable to summon entity due to duplicate UUIDs, yet setting the above command to run every tick from a datapack creates two entities upon loading the world.

Steps to reproduce:

1. Set above command to run every tick
2. Exit and reenter the world
3. Run

   execute as @e[type=minecraft:area_effect_cloud] run data get entity @s UUID

   Resulting in two entities reporting/having the same UUID. Note: only one entity is selected when directly selecting by UUID aec-0-0-0-1.