The bug

When using the /execute store command on a tipped arrow with custom potion effects, rather than replacing the respective NBT, it will duplicate the field first, THEN replace the respective NBT in the new field.

How to reproduce

1. Summon an arrow with CustomPotionEffects nbt data:

   /summon arrow ~ ~ ~ {CustomPotionEffects:[{Ambient:0b,ShowParticles:1b,Duration:1200,Id:26b,Amplifier:1b}]}
   

2. Run an example execute store command:

   /execute store success entity @e[limit=1,type=arrow] CustomPotionEffects[0].Id byte 1 run say store
   

3. Using data get on the entity results in duplicate entries:

   /data get entity @e[type=arrow,limit=1] CustomPotionEffects

   Result

   Arrow has the following entity data: { 
     CustomPotionEffects: [
       {Ambient: 0b, ShowParticles: 1b, Duration: 1200, Id: 26b, Amplifier: 1b}, 
       {Ambient: 0b, ShowParticles: 1b, Duration: 1200, Id: 1b, Amplifier: 1b}
     ]
   }
   

Using /data remove doesn't seem to modify the CustomPotionEffects tag at all.

/data remove entity @e[limit=1,type=arrow] CustomPotionEffects[0]

using this and then /data get gives the same results as listed above.