Steps to reproduce:
1. /give @p minecraft:diamond_sword{Enchantments:[{id:"minecraft:sharpness",lvl:10}]} 1
2. /give @p minecraft:diamond_sword{Enchantments:[{id:"minecraft:sharpness",lvl:10},{id:"minecraft:sharpness",lvl:10}]} 1
Test out both swords, notice how the second one does more damage.
3. /give @p minecraft:bow{Enchantments:[{id:"minecraft:power",lvl:10}]} 1
4. /give @p minecraft:bow{Enchantments:[{id:"minecraft:power",lvl:10},{id:"minecraft:power",lvl:10}]} 1
Notice how both bows do the same amount of damage.
Why is this marked as a duplicate? Sharpness 10 and Power 10 WORK! Putting sharpness 20 and power 20 on something also works! What doesn't work is Power 10 + Power 10, where as sharpness 10 + sharpness 10 works.