What causes the double hits on spirit blasts?

[AveryBlue]

Sometimes when I shoot the blast it'll tick twice and do double the damage but other times it just phases through the enemy and only hits them once.

[Darc Vader]

It’s usually knockback that does it I believe. The first hit can push them back and they take a second from being hit ‘again’

[Matcha]

I think it's depending on :
1. enemy can be knockback
2. distance between you and enemy
3. size of enemy

[Boksha]

Also Shaman Stone increases the size of the projectile, meaning there are more positions from where it'll hit twice instead of once.