Hajj is obligatory on every Muslim only once during his or her lifetime, provided that he or she is able to perform it. The order comes from Allaah in the ayah:
“Hajj (Pilgrimage) to the House (the Ka'bah) is an obligation that humanity owes to Allaah, for those who can find a way (i.e. for those who can find the means to do it). And whoever disbelieves (i.e. denies the Hajj), then Allaah is beyond any need of all that exists (from His creation).” [Surah Aali ‘Imraan: 197]
And His order in the ayah:
“The Hajj is in the well-known (lunar) months. So whoever intends to perform Hajj in these months, then he should not have sexual relations (with his wife), nor commit sin, nor argue unjustly during the Hajj.” [Surah Al-Baqarah: 197]
And in His order:
“Complete (perform properly) the Hajj and ‘Umrah for Allaah...” [Surah Al-Baqarah: 196]
From these previous ayaat we can conclude that Hajj is obligatory on every Muslim who has the (physical, financial, and circumstantial) ability to reach the sacred precincts of the Ka'bah at Makkah.
As for those who have the ability to do so but do not perform Hajj, then they are committing a major sin from the major sins of Islaam and, moreoever it is one of the things that indicate Kufr (Disbelief).
Therefore, Hajj is obligatory once in a person's lifetime – for those who have the ability to do it. Whoever performs it more than once, then these extra Hajj are considered voluntary (naafilah).