A specially built deck containing no duplicates of any card.
Singleton decks exist across a variety of card games due to cards with mechanics such as Shupe's Day Off and Radeyah that require the deck-building restriction of including no duplicates in order to proc. While these decks can be inconsistent and weaker than others due to the relative strength of the cards that must be included to ensure no duplicates, this disadvantage is paid off by the ‘highlander’ card (Shupe or Radeyah in Gwent) delivering a powerful effect to compensate.
Singleton decks are also known as highlander decks (after the Highlander franchise, which had the tagline ‘There can only be one’) as well as being called by the key card the deck is built around, e.g. Shupe NG or Radeyah Pincer Maneuver.