In some cases involving linked abilities, an ability requires information about "the exiled card." When this happens, the ability gets multiple answers. If these answers are being used to determine the value of a variable, the sum is used. For example, if Clive's Hideaway's hideaway ability triggers twice, two cards are eventually exiled. The last ability of Clive's Hideaway will allow you to cast both exiled cards without paying their mana costs (assuming you control four or more legendary creatures at that time.)