If an attack, for example, Shadow Rider Calyrex V's Astral Barrage says "Choose 2 of your opponent's Pokemon and put 5 damage counters on each of them", can you choose the same Pokemon twice and put 10 damage counters on it?
Usually, no. If the same Pokémon can be targeted multiple times by an attack, it will be stated in the attack (Such as here, in Unified Minds Alolan Raichu's Electro Rain attack)