D&D 5E Arm Blade + Repeating Shot Infusions

[Dogo]

So a PC asked if he could have an Arm Blade and have repeating shot on it. I’m not sure if I should allow it. Would it take up 2 Attunement slots or 1? Would this make the PC too powerful?

 

[Leatherhead]

have an Arm Blade and have repeating shot on it.

No.
Arm Blades are Magic weapons, infusions can only be applied to non-magic items.
Even if you ignore that:
No.
Arm Blades are Melee weapons, Melee weapons don't have the ammunition property, you would need the Returning Weapon infusion.

However, there used to be an item called an Armbow, which does almost exactly the thing that your player is describing. Having a custom infusion that acts Armblade but works with one-handed ranged weapons instead seems reasonable. Giving it the unlimited ammo power but not the +1 bonus of Repeating Shot should keep it more or less inline with other infusion options, including the normal Repeating shot and the Wand Sheath.

 

[Paul Farquhar]

So a PC asked if he could have an Arm Blade and have repeating shot on it. I’m not sure if I should allow it. Would it take up 2 Attunement slots or 1? Would this make the PC too powerful?

RAW, no. You cannot place an infusion on an item that is already magical.

In practice, I would probably allow it.

Mechanically, if the armblade is found rather than made by a PC artificer, it only requires one attunment slot no matter how many different magical properties it has.