Stacking Rabadons deathcap

It's a unique passive. You won't get the passive from it, just the pure 120 AP. But the passive from the first rabadon's will be affected by the 2nd rabadon's 120 AP.

If you just want more raw damage, you're better of buying one of these: : More penetration means more damage : Another AP ratio in your combo is often stronger than raw AP : Heyo, movement speed and burst damage?

Yep, just pure AP.

So if you're looking for the most straight AP the more efficient way to go is to build one Deathcap then build Spellbinders. You get the same AP plus movement speed for 2900 gold instead of 3600. The unique passives or actives only count once so its a cheaper way to go and you get the added bonus of extra MS.