League of Legends (LoL) Question: item's passives

No, the passives of Spirit Visage and Rabadon's Deathcap will not be doubled. This is due to the prefix UNIQUE. This means that even if you buy multiples of that item, the passives will not multiply.

A special case takes place when an UNIQUE passive is named, for example: Luden's Tempest first passive says the following: UNIQUE passive: Haste: 10% cooldown reduction. This passive will again not double if you buy multiple Luden's Tempest's. However, if you for example buy Everfrost when you already have Luden's Tempest, you will not gain the benefits of the UNIQUE passive: Haste on Everfrost since you got the same passive (Haste) from Luden's Tempest already .
(So you will only gain 30% Cooldown reduction when you buy these two items, instead of 40%).

Sometimes the UNIQUE named passives of some items share the same name, but have slightly different effects (e.g. Trinity Force and Iceborn Gauntlet or Sterak's Gage, Phantom Dancer and Maw of Malmortius). In these cases the strongest passive will apply.

EDIT: It is a bit vague when which passive will apply, it differs from (named) passive to passive. Your best bet is to go over to this page where every named passive is listed & in some cases which passive applies first.

So Tldr; UNIQUE passives are not doubled (if multiple of the same item are bought) and UNIQUE named passives are not doubled (even if they are on different items).

well my frend it will not