After updating to 1.21, if the custom attribute_modifiers of an item have the same UUID, only the last one will be retained.

How to reproduce:

1. Create a world in 1.20.6
2. Run the following command: 

   /give @p minecraft:diamond_sword[minecraft:attribute_modifiers={modifiers:[{name:"a",uuid:[I;1683709482,44580970,-1540485464,1994649380],amount:6,operation:add_value,slot:mainhand,type:"minecraft:generic.attack_damage"},{name:"a",uuid:[I;1683709482,44580970,-1540485464,1994649380],amount:-2.4,operation:add_value,slot:mainhand,type:"minecraft:generic.attack_speed"}]}]

3. Observe the attribute_modifiers of the item:
   → +6 attack damage -2.4 attack speed
4. Upgrade the world to 1.21
5. Observe the attribute_modifiers of the item:
   → -2.4 attack speed
6. Run the command:

   /give @p minecraft:diamond_sword[minecraft:attribute_modifiers={modifiers:[{id:"minecraft:645b5e2a-02a8-406a-a42e-0ea876e3ef24",amount:6,operation:add_value,slot:mainhand,type:"minecraft:generic.attack_damage"},{id:"minecraft:645b5e2a-02a8-406a-a42e-0ea876e3ef24",amount:-2.4,operation:add_value,slot:mainhand,type:"minecraft:generic.attack_speed"}]}]

   → Both attribute_modifiers appear, indicating that using the same ID is allowed.