When a spell has multiple targets, can I choose the same target multiple times?
Is there any general rule I can refer to?
An example of ambiguity is the spell Steel Wind Strike:
You flourish the weapon used in the casting and then vanish to strike like the wind. Choose up to five creatures you can see within range. Make a melee spell attack against each target. On a hit, a target takes 6d10 force damage.
You can then teleport to an unoccupied space you can see within 5 feet of one of the targets you hit or missed.
Nowhere does it say you have to choose up to five different creatures, although Crawford has disallowed targeting the same creature multiple times on Sage Advice EU. However, this answer does not give light on a general rule.
Similarly, for Acid Splash:
You hurl a bubble of acid. Choose one creature within range, or choose two creatures within range that are within 5 feet of each other. A target must succeed on a Dexterity saving throw or take 1d6 acid damage.
The intended answer is clearly no. But how can I know that? Is there a general rule?
Best Answer
Only if the spell says so
There are spells that can direct multiple attacks against the same target, but they use different language. Instead of saying to choose a number of targets, such a spell says that it creates a number of attacks (or strikes, or missiles, etc.) and then requires you to choose a target for each attack. Some spells that use this language are magic missile, scorching ray, and eldritch blast. For example, here is the language used for magic missile:
Note that the spell is worded in such a way that the same number of darts is created regardless of how many distinct targets you choose. The last clause makes it explicit that multiple darts can be directed at the same target, but even the first 2 sentences by themselves are sufficient to establish this.
As for your specific examples:
Neither steel wind strike nor acid splash can attack the same creature twice
The targeting clause for steel wind strike is:
You choose the targets first, and then you get to make one attack against each chosen target. If you only choose a single target, you only get to make a single attack. The same logic applies to acid splash: if you target a single creature, then you are not targeting 2 creatures. Acid only splashes each target once.
Any other interpretation leads to absurd results
If you rule that effects like these can choose the same target multiple times in order to stack multiple effects on that target, that would make the Hunter Ranger's Whirlwind Attack infinitely powerful:
Just choose to attack the same creature 1000 times. It says any number, right? Even better, choose every creature in range 1000 times and instantly kill them all. Clearly this is not a reasonable ruling. There are certain to be other ridiculous edge cases like this if you allow multiple targeting of the same creature in this way.