To reproduce:

Step 1
In the spawn chunks run this command

/summon minecraft:armor_stand ~ ~ ~ {UUID:[I;2038625872,-790803325,-2057907387,1487988512]} 

Step 2
Go to the nether and run these commands

/forceload add ~ ~ 

/summon minecraft:armor_stand ~ ~ ~
{UUID:[I;2038625872,-790803325,-2057907387,1487988512]} 

Expected: "UUID of added entity already exists"
Actual: "Summoned new Armor Stand"

Step 3
Run these commands

/execute as 7982f650-d0dd-4c83-8556-d34558b0e720 run say hi 

Expected: one armor stand responds
Actual: both armor stands exist and respond

/data merge entity 7982f650-d0dd-4c83-8556-d34558b0e720 {ArmorItems:[{},{},{},{id:"dirt",Count:1b}]} 

Expected: both armor stands have their head slot set to dirt?
Actual: only the armor stand in the overworld is modified, regardless of which armor stand was summoned first