Liches are, obviously, undead. The clone spell specifies that it creates a "living duplicate" of the caster as a sort of safeguard. So my question is, if a lich casts Clone, does it create a duplicate of the lich when it was alive, or a duplicate of its undead body now?

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I would assume a lich casting clone just gets a duplicate lich body.  Note per the spells says   "you can also choose to have the clone be a younger version of the same creature.".  A living human being is a different type of creature than a lich.

RAW the target needs to be living: "This spell grows an inert duplicate of a living, Medium creature as a safeguard against death." 

Technically gnomes and halflings (and other small humanoids) cant be cloned either.

Quote from HailRobonia >>

RAW the target needs to be living: "This spell grows an inert duplicate of a living, Medium creature as a safeguard against death." 

Technically gnomes and halflings (and other small humanoids) cant be cloned either.

 Guess it depends on how your DM defines it. "undead", could technically be living. Up to DM discretion

Yo, Imagine being living, making a clone of yourself, undergoing the ritual to become a lich, what happens? Would the soul be forced into the undead form and when the soul is freed it becomes willing and comes back to life? Off-topic but interesting non the less. 

It is definitely a 'talk to your dm' question. I would personally say no, because a lich is specified as undead and the spell requires that the target be living, thus unable to meet the requirements of the spell. Plus liches have alternate means to live beyond their redeath. 

Per JC: "In D&D, "living creature" means a creature that isn't dead, regardless of the creature's type."  

Therefore, a Lich with positive hit points is considered "living".  

A Lich would never waste the time to cast Clone because their phylactery effectively gives them the same benefit that the Clone spell would theoretically give them. Or at least that’s how thing work in my campaigns. 

Quote from Tim >>

A Lich would never waste the time to cast Clone because their phylactery effectively gives them the same benefit that the Clone spell would theoretically give them. Or at least that’s how thing work in my campaigns. 

 No i agree. To a creature like a lich it's useless. Merely asking if it's possible by RAw

RAW, Yes.

Since undead and liches specifically are considered to be a medium living creatures they could cast clone. That being said, it would only work assuming that the liches phylactery is destroyed since the soul must be able and willing to return to the body for clone to work and a liches phylactery traps the soul within it.

This happening would make you a lich without a phylactery and basically just a highly magical undead.  

Quote from Gh0styy >>

Liches are, obviously, undead. The clone spell specifies that it creates a "living duplicate" of the caster as a sort of safeguard. So my question is, if a lich casts Clone, does it create a duplicate of the lich when it was alive, or a duplicate of its undead body now?

A Lich can certainly cast Clone, but Clone simply doesn't duplicate the caster - it duplicates the target, and the spell is Range Touch.  Are you asking if a Lich can be targeted with Clone, and if so, what happens?

Clone can only target a "living, Medium creature".  "Living" means "not dead", and a Lich that can cast a spell isn't dead, so it's living, just as much as a fire elemental is living.  Provided the Lich is also Medium-sized, it can be targeted with Clone, and the spell will make the duplicate.  The exact specifics of the inert clone are broadly speaking up to DM fiat, but they only matter between casting the spell and dying - if the original creature dies, the clone's physical form is overwritten with the cloned creature's physicality anyway (which is why you can cast Clone, level up, and then die, and you'll respawn per the Clone spell with your new level intact).  Within that window where it could matter (maybe someone is looking at the clone somehow), the clone is, caster's choice, the spell's target or a "younger" version, whatever your DM wants that to mean.

I have a lich npc do that, because an old body means they can't run to the entrance ceremony fast enough lol.

This spell grows an inert duplicate of a living creature as a safeguard against death.   Undead are by default, not alive. 

RAW means it wouldn't work.  Odd exceptions may apply, and the DM is free to make cool narrative rulings if they wish.

The mechanical/RAW answer to this question has been answered by ConaltheGreat and quindraco completely. Wyldeshot has, in my opinion, given a narrative answer. Lich are living from a mechanical standpoint and if they were not, they would no longer be creatures, but objects. I believe we create a rat king of a problem for ourselves if we try to make the case that a lich cannot cast clone because it is not alive in the same way that a human or elf is alive. 

From Monsters, Type:  Undead are once-living creatures brought to a horrifying state of undeath through the practice of necromantic magic or some unholy curse. Undead include walking corpses, such as vampires and zombies, as well as bodiless spirits, such as ghosts and specters.

Liches are a monster, typed Undead.  The rules clearly state that Undead are not alive.  Thus they cannot be targeted by any spell requiring it to be alive.  Its mechanically pretty clear cut.  The rules are right there.

Quote from Wyldeshot >>

From Monsters, Type:  Undead are once-living creatures brought to a horrifying state of undeath through the practice of necromantic magic or some unholy curse. Undead include walking corpses, such as vampires and zombies, as well as bodiless spirits, such as ghosts and specters.

Liches are a monster, typed Undead.  The rules clearly state that Undead are not alive.  Thus they cannot be targeted by any spell requiring it to be alive.  Its mechanically pretty clear cut.  The rules are right there.

This is the logical fallacy of equivocation in an attempt to avoid being perceived as wrong. 'Living creature' has no special meaning in D&D per JC, and when the phrase has appeared it was a mistake by his own admission. A 'living creature' is a creature. Undead are creatures. Constructs are creatures. A warforged is a creature. Monsters are creatures. They all would qualify for the clone spell provided they have the material components necessary to cast it (1 cubic inch of flesh). Not all undead, constructs, monsters, and odd humanoids have it.

Once again, your answer is a narrative answer, not a mechanical one. It is not necessarily wrong, just not correct for the forum you are in.

To borrow your own considerate words, the rules are right there.

One thing I'd like to add. The "medium" creature constraint that is mentioned earlier in the thread, has been errata'ed out. You can now clone gnomes, halflings, faeries or any other living creature.

"This spell grows an inert duplicate of a living creature as a safeguard against death."

[New] Clone (p. 222). In the Components entry, “hold a Medium creature” is now “hold the creature being cloned.” In the description’s first sentence, “a living, Medium creature” is now “a living creature”. In the description’s second sentence, “a sealed vessel” is now “the vessel used in the spell’s casting.”

Again... undead aren't alive.  Its been in cannon since D&D started, and in every edition.  It is clear here too.  But again, if you want to let a lich in your game cast the spell, more power to you.  A DM can break the rules as they see fit.  Have fun!

Quote from Wyldeshot >>

Again... undead aren't alive.  Its been in cannon since D&D started, and in every edition.  It is clear here too.  But again, if you want to let a lich in your game cast the spell, more power to you.  A DM can break the rules as they see fit.  Have fun!

Canon and previous versions have exactly zero relevance to the current edition of the game. A DM is free to break the rules. They are also free to fail in understanding them and thank you for offering yourself up as one such example. This forum is not about what a DM can do however, it is about what the rules are. Your chosen definition of 'alive' has no mechanical bearing on the game and an explanation was given for why well before either of us posted. A lich is a 'living creature' through the lens of the rules of the game. The cessation of biological functions are immaterial to this discussion in a game where creatures can continue to 'live' without them. As you say though, you are free to ignore and break whatever rule you wish.

Casting clone doesn't result in a carbon copy of a creatures stat block. 

It grows - one must assume from genetic materials - a body. 

So you cannot clone elementals. They don't have genetics, and more over, they don't have a biology and they don't grow. They're just flame that behaves as is alive.

And liches aren't alive, except in the sense that they're not dead - meaning removed from play. Undead die, and are thus dead. They are then reanimated, which doesn't restore them to life. See Ressurection or Raise dead if in doubt. 

It is unfortunate that the rules give a specific definition of 'alive' for players to cling to when making incorrect rules interpretations. It's typical for discussions like this that people who'd swear in court that they 'know the rules' propably do, very well even, but refer to only that particular snippet of text in the rule books that supports the view they like.

Here's another bit of rules: 

Undead are once-living creatures brought to a horrifying state of undeath through the practice of necromantic magic or some unholy curse.

Ok I read through the posts and once again there is just enough inclarity for folks to have an argument over interpretations. So each DM will have to decide what happens in their world. That said coral raised an interesting side issue - what happens if the mage casts the clone spell before ndergoing the transition to lichdom? I am assuming that the transition rite forces the mage’s soul into the phalctery so it can’t go to the clone body. If the Lich and it’s phylactery are then destroyed does the soul go to the clone resurrecting the living mage ( possibly as a much younger version) or what?