Is it permissible for a person to make a will that their heirs perform Hajj on their behalf, and what is the ruling if they had already performed Hajj previously?

One who is able, and has not previously performed Hajj, must perform it immediately. This is the view of Malik, Ahmad, and Abu Hanifa. It is not permissible for him to be negligent concerning this obligation and then recommend that his heirs perform Hajj on his behalf while he was able to perform it during his lifetime, because Allah says: "And Hajj (pilgrimage to Makkah) to the House (Ka’bah) is a duty that mankind owes to Allah, those who can afford the expenses (for one’s conveyance, provision and residence); and whoever disbelieves [i.e. denies Hajj (pilgrimage to Makkah), then surely Allah is Rich, free from all needs of the ‘Alamin (mankind, jinn and all that exists)]."