Rule 701.20c

A card that is currently revealed may be revealed again. Example: Telepathy is an enchantment card that reads “Your opponents play with their hands revealed.” Silvergill Adept is a creature card that reads, in part, “As an additional cost to cast this spell, reveal a Merfolk card from your hand or pay {3}.” A player may reveal a Merfolk card from their hand to pay the additional cost of Silvergill Adept even if that card is already revealed due to Telepathy’s effect.