Stacking same enchantment multiple times on a bow does not increase it's effects.

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.