This spell quite broken if wielded properly.  2 Methods I could think of, the former essentially what other's have posted.  The latter allows an earlier exploitation.   

1. Wizard casts Simulacrum --> Simulacrum #1 (Without a 7th Level Slot), *Requires at least one "additional" castable slot (7th, 8th, or 9th): Minimum Level 15*
2. Sim #1 casts Simulacrum (8th Level Slot)--> Copy of Wizard [Post Long-Rest](Simulacrum #2) <ie a Wizard with all of his spell slots>
3. Sim #2 casts Simulacrum (7th or 8th Level Slot)--> Copy of Wizard [Post Long-Rest] (Simulacrum #3) - Rinse and Repeat. (At the cost of 1500 gp of Powdered Ruby
4. Expl: The wizard is a Humanoid, so this is castable as long as you have the components.  Only the initial Requires an 8th Level slot.  

Technically you can do this at Level 13, but it requires "Glyph of Warding", Specifically <Spell Glyph>.  Store Simulacrum in a Spell Glyph.  Trigger: Creature/Caster being copied standing on the Glyph.  

1. Cast Simulacrum into Spell Glyph (7th Level)
2. Take Long Rest. 
3. Stand on Glyph.  (Simulacrums with all spell slots).  Each new Simulacrum can now Create Spell Glyphs that target the Caster. 
4. Caveat, it costs an add'l 200gp per casting, per the Glyph of Warding Material Components.  Honestly finding enough ruby, powdered diamond and Ice could prove difficult if your DM wanted to stop the exploitation.  Honestly, I would only use them to do research and protect my "Stronghold".  

Heaven help if you have a game last until 18, for Simulacrums to get Spell Mastery.  (Level 0, 1, and 2 Spells At-Will, ie ).  Note: Pretty sure the Capstone (Signature Spells requires the user to regain 'spell energy/slots' so doesn't work.  So level 13 and 18 is where the 'magic' happens.  

 Hahahahahahaha!!

INFINITE WISHES!!!!

“You shape an illusory duplicate of one beast or humanoid...”

As a DM I read “If you cast this spell again...” and feel that a Simulacrum of yourself would also be considered ‘you’, therefore destroying both itself AND the newly created Sim2. Say goodbye to 3000gp in ruby dust and 24 hours :-)

Bonus -  Using Nystul’s Magic Aura for 30 days allows you to Mask a willing creature’s type to appear humanoid/beast and create a Sim of anything. If you have the skill/patience to become friendly with it at least...

Quote from DivineAzazel >>

Technically you can do this at Level 13, but it requires "Glyph of Warding", Specifically <Spell Glyph>.  Store Simulacrum in a Spell Glyph.  Trigger: Creature/Caster being copied standing on the Glyph.  

1. Cast Simulacrum into Spell Glyph (7th Level)
2. Take Long Rest. 
3. Stand on Glyph.  (Simulacrums with all spell slots).  Each new Simulacrum can now Create Spell Glyphs that target the Caster. 
4. Caveat, it costs an add'l 200gp per casting, per the Glyph of Warding Material Components.  Honestly finding enough ruby, powdered diamond and Ice could prove difficult if your DM wanted to stop the exploitation.  Honestly, I would only use them to do research and protect my "Stronghold".  

Heaven help if you have a game last until 18, for Simulacrums to get Spell Mastery.  (Level 0, 1, and 2 Spells At-Will, ie ).  Note: Pretty sure the Capstone (Signature Spells requires the user to regain 'spell energy/slots' so doesn't work.  So level 13 and 18 is where the 'magic' happens.  

 Glyph doesn't work that way. You expend one slot for the appropriate level glyph (at least lvl 7) AND one slot for the spell you want to store (lvl7 simulacrum). You either need a lvl 8 slot for this as well, being lvl 15 like above, or you need to be lvl 20 for two lvl 7 slots.

Glyph of Warding

[...]

Spell Glyph. You can store a prepared spell of 3rd level or lower in the glyph by casting it as part of creating the glyph. [...]

At Higher Levels. [...] If you create a spell glyph, you can store any spell of up to the same level as the slot you use for the glyph of warding.

The Sim is definitely its own creature, even though it goes on your turn and obeys your every spoken command.

The issue I hadn't thought of before today was the "long rest while it casts on you so the next Sim has maximum firepower" problem.

As a DM, letting a wizard do this ad nauseum with whatever wealth I may have bestowed... eventually, those Sims will meet delayed blast fireballs, disintegration, or -- heavens forbid -- a meteor swarm that would fart in the mouth of 30,000gp of rubies and 20 full days of work (because if you do the long rest thing, you have to take the better part of the whole day in other activities to get the next long rest benefit).

But if you're an elf and do stationary work for 8 hours and then take your trance... oh no... that's 12 hours... the Sim machine continues!

Level 17 wizard casts this on themselves, creating a simulacrum with zero 7th level slots and one 9th level slot. That simulacrum uses its 9th level slot to cast Wish, in order to cast Simulacrum targeting the original wizard (the one who still has a 9th level slot). This creates another simulacrum in just one action, without any cost. Seeing as the original wizard never used its 9th level slot, the second simulacrum also has a 9th level slot with which to cast Wish. Bam, free copy of yourself every six seconds. So I think the simulacrum would have to destroy itself by casting Simulacrum to avoid this loophole.

Quote from ElvishJerricco >>

Level 17 wizard casts this on themselves, creating a simulacrum with zero 7th level slots and one 9th level slot. That simulacrum uses its 9th level slot to cast Wish, in order to cast Simulacrum targeting the original wizard (the one who still has a 9th level slot). This creates another simulacrum in just one action, without any cost. Seeing as the original wizard never used its 9th level slot, the second simulacrum also has a 9th level slot with which to cast Wish. Bam, free copy of yourself every six seconds. So I think the simulacrum would have to destroy itself by casting Simulacrum to avoid this loophole.

This spell has a 12 hour casting time. Not sure how you're concluding you can cast it in one action. 

What if you made a simulacrum army, had them get into a line ad wish for the next wish to be stronger. This would probably get pretty strong because a powered up wish would make another wish even more powerful. Eventually this would just kill the caster after it was cast, but you can still do some crazy things.

Quote from wordflyerjosh >>

Quote from ElvishJerricco >>

Level 17 wizard casts this on themselves, creating a simulacrum with zero 7th level slots and one 9th level slot. That simulacrum uses its 9th level slot to cast Wish, in order to cast Simulacrum targeting the original wizard (the one who still has a 9th level slot). This creates another simulacrum in just one action, without any cost. Seeing as the original wizard never used its 9th level slot, the second simulacrum also has a 9th level slot with which to cast Wish. Bam, free copy of yourself every six seconds. So I think the simulacrum would have to destroy itself by casting Simulacrum to avoid this loophole.

 

This spell has a 12 hour casting time. Not sure how you're concluding you can cast it in one action. 

 Wish literally cats everything in one action

I am really curious to know on how Pumat Sol managed to create more than one Simulacrum.

Homebrew spell version, maybe Level 9?

Can a Simulacrum cast Magic Jar?

Pumat stated that he had assistance from the Cerberus Assembly.
https://criticalrole.fandom.com/wiki/Pumat_Sol

That entirely depends on whether a Simulacrum has a soul as that's how Magic Jar lets you possess people. Most constructs aren't en-souled creatures but obviously there are exceptions like warforged. Really it's DM fiat but I'd say sure, why not. RAW this spell states the way a simulacrum is different from the original creature and that otherwise it's the same. So if the original has a soul then so must the copy.

Guys you're all trying to cast this to copy yourselves directly, having your sim lose out on its 7th-level spell slot (or 9th level if you wish to cast without cost). Have you considered the possibility of casting this to copy another  spellcaster with wish and/or simulacrum? This way they could have all spell slots available, and your simulacrum of them could copy you after you finish a long rest, and that copy of you can copy your friend, repeat cycle. Then each simulacrum is only missing either its 7th OR 9th level spell slot AFTER having made the next. 

Even if you're the only wizard in your party, this still works as long as someone else knows wish.

Its until dispelled?! That’s so overpowered!

You need both. The material component is part of the creature's body, while the first line of the spells says "You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell"

Quote from Leerjawise >>

if you are within touch range of a tarrasquefor 12 hours...

 They are known to hibernate for long periods of time so it could be asleep while you cast it

Prerequisite: Silmacrum, Wish. Cast Simulacrum copying yourself. Your simulacrum casts Wish as Simulacrum copying you. Their Simulacrum casts Wish as Simulacrum copying you. Infinite loop. You still have control over each one, by sending commands down the chain. OP

If you have multiple wizards capable of casting this, they could each cast Simulacrum on a single target and get multiple Simulacra of that target to exist concurrently, since the text of the spell says 'If you cast this spell again, any duplicate you created with this spell is instantly destroyed'  That would leave multiple casters unable to cast it again without destroying the simulacrum they made, but a particularly powerful king or cult leader could make this happen. It also is why there being four Pumats is possible. (under the interpretation of the simulacra still counting as "you" for the purposes of the spell)