Each Card is allowed only once.
Highlander is a casual constructed variant of Magic which allows only one copy of each card in the deck, with the exception of basiclands. The name Highlander is a reference to the movie of the same name, whose tag line was “There can be only one”. 
100 Card Minimum.
A deck must contain at least 100 cards.
Any card from Alpha up through the latest Magic set can be used (except for banned cards). Cards from the Portal sets, International Collector Editions, and Championship decks can be used provided you put all your cards in opaque sleeves and the International Collector Edition card edges are rounded.
All new Magic expansions become legal on the worldwide release date of the edition, as in all sanctioned DCI formats.