There is consensus among Muslim scholars that Hajj is obligatory only once during the lifetime of a Muslim, unless someone vows to perform extra Hajj, in which case he must fulfil his vow. Whatever is done over and above is supererogatory or optional.
Abu Hurairah narrates that once, in a sermon, the Prophet (peace be upon him) said: “O people! Allah has prescribed Hajj for you, so you must perform it.” A man asked: “Every year, O Prophet of Allah?” The Prophet (peace be upon him) remained silent. When the man repeated it thrice, the Prophet (peace be upon him) said: “Had I said ‘yes’, it would have become a yearly obligation, and this would have been beyond your power.” Then he added: “Leave me alone so long as I leave you alone (i.e. do not pester me with questions about things which I omit and do not mention). Some people who lived before you were destroyed because they asked too many questions and disagreed with their Prophets. So when I command you to do something, you must obey and do it to the best of your power, and if I forbid you from something, then avoid it.” (Bukhari and Muslim)
Ibn ‘Abbas narrates that once the Prophet (peace be upon him) addressed us and said: “O People! Hajj has been prescribed for you.” At this Al-Aqra’ bin Habis stood up and asked: “O Prophet of Allah! Are we to perform Hajj every year?” The Prophet (peace be upon him) said: “Had I said ‘yes’, it would have become a (yearly) obligation, and had it become a (yearly) obligatory duty you would have failed to keep it. Hajj is obligatory only once in one’s lifetime. Whatever one does over and above this is supererogatory (a voluntary act) for him. (Reported by Ahmad, Abu Daw’ ud, Nasa’i and Al-Hakim, who regards it a sound hadith)