Here is the wording of Ride Against the Enemy for reference:
Since the character card is only “revealed” and never actually “played,” it would seem that characters would be revealed regardless of uniqueness of other cards in play. However, this leads to problematic situations if the character is then defeated and placed in an opponent’s marshalling pile while still being in play or eliminated elsewhere. And clearly a more thematic approach would be for a hazard creature version of Elrond to not be able to attack himself when he’s already in play!Playable on a company moving through a Wilderness if you have a character in your hand. Reveal the character. A single-strike hazard creature attack is made on the company with the attributes of the revealed character, except the prowess is increased by 7. Other cards have no effect on this attack. The attack is detainment if the revealed character and the company are both minion or both hero. If defeated, place the character in your opponent’s marshalling point pile— he receives the character’s marshalling points as kill points. Otherwise, discard the character.
With all of that in mind, this committee is issuing the following errata for Ride Against the Enemy:
This means that no, you cannot reveal a unique character with Ride Against the Enemy if the character is already in play for the purposes of uniqueness (including already being eliminated).Card Erratum: Change active condition to «Playable on a company moving through a Wilderness if you have a character in your hand that is not a manifestation or another copy of a unique card in play.»