The bug

Lightning sometimes does damage multiple times, instead of only once (which would be the expected behavior).

To reproduce

• Summon a zombie pigman
• Summon a Lightning bolt nearby with /summon minecraft:lightning_bolt
  The pigman will sometimes take damage twice.

Code analysis

Based on 1.11 decompiled using MCP 9.35 rc1

Lightning bolts have two fields which are important for the following:

• lightningState: Initialized to 2
• boltLivingTime: Initialized to a random integer between 1 and 3 (both inclusive)

Every time a lightning bolt is updated it does the following:

1. Reduce lightningState by 1
2. If lightningState < 0
   1. If boltLivingTime: Set dead
   2. Else if lightningState < random integer between -9 and 0 (both inclusive)
      1. Reduce boltLivingTime by 1
      2. Set lightningState to 1
3. If lightningState >= 0: Deal damage to entities

This means a lightning bolt can try to deal damage to entities up to 8 times with the last strike being in the 35th tick after the lightning bolt was created. This allows it to deal 20 damage to a player which would kill him.