Double Shot is the 3rd upgrade on Path 1 for the Ninja Monkey in Bloons TD 6. This upgrade allows the Ninja Monkey to throw 2 shurikens per shot instead of 1. Both shurikens pop 4 bloons per shot each and are simultaneously shot at a rate of every 0.42s at a slight angular deviation from each other.
The Double Shot Ninja is not a particularly strong tower overall, although with the Seeking Shuriken crosspath it is decent at carrying the early-game before Rainbow Bloons. Its main use is as a stepping stone for Bloonjitsu, or in some scenarios as a purely early-game advancement. Most of the Double Shot's power will come with its crosspaths, each with their own specializations for the Ninja Monkey. And just like Bloonjitsu, a Double Shot Ninja is a good option to pair with Berserker Brew specifically for the extra damage and pierce.
For Path 3, Seeking Shuriken is great for optimize each of its shurikens, allowing the shuriken attacks to easy essentially deal 2 damage per shot from each 1-damage shuriken, while Caltrops can be added to give the Ninja additional popping power via an additional caltrop attack. For Path 2, Distraction and Counter-Espionage can assist with difficult rounds in the mid-game.
Caltrops is a strong crosspath option as the Caltrops deal decent overall damage.
Distraction and Counter-Espionage are still good options, however Distraction is luck-based and a Double Shot with Counter-Espionage isn't reliable at grouped bloons. They can be used to assist with rounds where a small amount of Camo Bloons or strong bloons (mainly Ceramic Bloons) are present, such as Round 40, where the Ninja can be used to distract the Ceramics or Round 51, where the Ninja can decamo and distract the Ceramics.
Do add damage buffs such as Berserker Brew nearby, as it will allow the shurikens to effectively deal much greater damage and popping power than if the same Berserker Brew were to buff a different tower.
Double Shot did not receive any balance changes since it was first released in BTD6. However, it can no longer have both Distraction and Seeking Shuriken at the same time, as those upgrades are mutually exclusive as crosspaths for this upgrade.
BTD6 upgrade icon in upgrades menu
Crosspathing in BTD6 (top path 2, bottom path 3)
Double Shot Ninja throwing two shurikens
3-0-1 Ninja Monkey throwing seeking shurikens
BTD6 upgrade icon
Instamonkey icon for Double Shot
In BTD6, the Double Shot ninja looks identical to the BTD5 Bloonjitsu Master.
When not under range of discount-based towers (Monkey Business, Monkey Commerce, etc), the Double Shot upgrade in Impoppable with the Cheaper Doubles MK ($1020 - $100 = $920) costs the same as the Double Shot upgrade in Hard without the Cheaper Doubles MK ($920).
It is currently the only tier 3 upgrade that shares the name with another tower. Specifically, the Monkey Buccaneer's Double Shot.