I have a quick question about the Twinned Spell feature.

I get that I can use it to hit two targets instead of one with a spell that otherwise only hits one target, but... can I have the same creature/foe/monster/being be both targets?

Basically, do I have to target two different creatures/foes/monsters/beings, or can I use it to double attack one?

Two different targets.  Relevant text is "a second creature."

I disagree. A "second creature" is not the same as a "different creature," in my opinion. As always, it comes down to the DM's discretion, but mine have always allowed targeting the same creature twice. 

I have a D&D session tonight, so I'll ask my DM what he thinks.

But our party seems to be heading toward a confrontation with what I think is a young dragon, and as my character is only a freshly-leveled-up level 3 sorcerer (and this is my first time playing D&D), I was trying to figure out how to do one hit with as much damage as I could.

It occurred to me that If I could cast Ray of Sickness in a level 2 spell slot and twin it with both hitting the dragon, I could do 6d8 poison damage, which to me sounds impressive for a level 3 sorcerer. Of course, that's assuming the twinned attacks can both hit one dragon. And if I fail the attack roll... then I've spent half my 2nd level spell slots and 2/3 of my sorcery points on a whiff. Of course, I'm thinking about throwing in "Tides of Chaos" to give me advantage on my attack and spend my last sorcery point to empower my damage.

Quote from jdahveed >>

I disagree. A "second creature" is not the same as a "different creature," in my opinion. As always, it comes down to the DM's discretion, but mine have always allowed targeting the same creature twice. 

Semantics aside, RAI and Sage Advice  indicate that the same creature is not a second creature.  

https://www.sageadvice.eu/2017/04/21/can-twinned-spell-target-the-same-person-twice/

(Obligatory "you don't have to care what the lead designers say")

As a DM I wouldn't allow it.  Twin is already arguably the strongest metamagic already without making it strictly better than Heighten or Empower.

Well, I talked with my DM. He said that, officially, twin spells can only be used with two different targets. But then he leaned in close and quietly said, "But I'd probably allow it."

So I'm not even going to try to use it unless it is an emergency and hope he's still OK with it.

In other news, our party either delayed or completely avoided (a bit early to tell which) the confrontation with the dragon that I was worried about.