R4-D1 was one of the astromech droids Jedi Master Darrus Jeht used on his Delta-6 starfighter during the Clone Wars, the other being R4-D2. Like other droid pairs assigned to Delta-6 prototypes, the two droids had a habit of arguing with each other. The two droids could also overwhelm the fighter's systems with input if they both tried to communicate at once, making the vessel unstable.
For these reasons, the Delta-7 starfighters which replaced the Delta-6 used only one astromech. Despite this, Jeht kept flying his prototype using both droids.