Worms that hurt themselves with or before their first Shotgun round still get a second shot
From Worms Knowledge Base
The way the game handles self-inflicted damage when the Shotgun is equipped is a little strange, resulting in two known glitches.
The first allows the worm using the Shotgun to still take their second shot, even if they are close enough to the target to have received splash damage.
The second means that, if the worm takes fall damage or lands on a mine while the Shotgun is equipped, the game considers that to be their first shot and still allows them to take a second.
The first bug can be exploited as a means of doing further damage to an enemy worm if, for instance, your worm has very little HP remaining. By aiming at an enemy worm but remaining within the splash zone, you can cause as much as 50 HP damage to your enemy worm, plus the damage caused by the second shot depleting your worm's last dribble of HP. This may only be practical if the target is the last worm remaining of the opposition.
The second bug allows you to traverse a larger area of the map to get one (1) Shotgun shot in, removing obstacles or uncertainty caused by taking a shot long-distance at the cost of some HP. Again, there may be few scenarios where this can be described as an advantage.