When dealing with multiple items with the Max HP attribute, only the latest change in max HP is performed.
For example, I have a helmet that gives +16 HP (+8 hearts) and a chestplate that gives +12 HP (+6 hearts). When I equip the helmet, I gain 8 hearts, as normal. But when I then equip the chestplate, the max HP from the helmet is cancelled out, leaving me with only 6 extra hearts, instead of 10.
Now, when I hold a sword which gives +8 HP (+4 hearts) my HP is set to only 14, despite wearing armor that should be giving more. Furthermore, when I stop holding it, all health boosts are completely cancelled, leaving me at 10 hearts of health despite wearing armor that should give 10 more hearts
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Wrong use of command. UUIDMost and UUIDLeast must not be same.
For example, use:
/give @p diamond_chestplate 1 0 {AttributeModifiers:[{AttributeName:generic.maxHealth,UUIDLeast:1,UUIDMost:1,Amount:10.0,Operation:1,Name:health1}]}
If you were to give yourself another item with the same command as above, changing the item name of course, you would also have to change the UUIDLeast and UUIDMost so the item does not match to the UUIDs from the previous armor to allow the armor modifiers to be combined.