If I cast the Simulacrum spell, and then use the Twinned Spell metamagic to cast it again, targeting myself and the first simulacrum, is the first simulacrum destroyed? What happens?
You can't cast Simulacrum on a Simulacrum.
The scenario as written doesn't work, because of the following two lines:
You shape an illusory duplicate of one beast or humanoid ...
... the illusion uses all the statistics of the creature it duplicates, except that it is a construct.
You are presumably a beast or humanoid as the caster of the spell, but when you go to cast the second version, you can't target your simulacrum since it is a construct.
Assuming that you instead try to target yourself and someone else with a twinned simulacrum...
You can probably use Twinned Spell with Simulacrum, but you might get a horrific chimera monster. Ask your DM.
Unfortunately, the rules for what can or can't be affected by the Twinned Spell metamagic is a bit fuzzy. Simulacrum doesn't have a target of self, so that's one hurdle cleared, but the rules for what constitutes a "target" of a spell is much less clear.
Personally, I would argue that Simulacrum does have one target (the creature to be copied), and as such you can use it with Twinned Spell, but there's another argument that the spell also targets/affects the Simulacrum itself, or potentially targets anything that the Simulacrum then interacts with, etc.
Assuming that Simulacrum can be twinned, there's still the question of what effect that has. Twinned Spell says that the spell will now affect two targets, but that doesn't necessarily mean that two simulacra are created. By a strict reading, the text is essentially modified to:
You shape an illusory duplicate of two beasts or humanoids that are within range for the entire casting time of the spell.
So, two targets, but you're still only making one simulacrum. Which then raises the question of what said simulacrum will look like. Having two "originals" makes a lot of the resulting text into nonsense. For example, the simulacrum will appear the same as both originals at the same time, have half the HP of each (not half added together, but both numbers simultaneously), the statistics of both, etc.
With a more liberal reading, a reasonable interpretation would be that the resulting simulacrum ends up as something in between the two original targets. Exactly how that would work and what attributes the creature gets from what original is essentially up to the DM.
But it's just as valid for the DM to rule that the resulting creature is a horrific, reality-warping horror that destroys the minds of those who look upon it as their feeble minds attempt to reconcile the duality of it's existence.