/summon entity ~ ~ ~ {UUID: [I; x, x, x, x]}
where x is a number divisible by the rarity (2048 by default)
for example:
/summon minecraft:cow ~ ~ ~ {UUID: [I; 0, 0, 0, 2048]}
/summon minecraft:cow ~ ~ ~ {UUID: [I; 0, 0, 2048, 0]}
/summon minecraft:cow ~ ~ ~ {UUID: [I; 0, 0, 0, 4096]}
you need a different UUID each time afaik