Only when the spell says it
There are spells that can target multiple attacks on the same target, but use different languages. Instead of saying that you should select a number of targets, such a spell means that it generates a number of attacks (or strikes or missiles, etc.) and you then have to select a target for each attack. Some spells that use this language are magic missile. scorching beam, and Eldritch explosion, For example, here is the language that is used for magic missile::
You create three glowing arrows of magical power. Each arrow hits a creature of your choice that you can see within range. An arrow deals 1d4 + 1 power damage to its target. The arrows all hit at the same time and you can tell them to hit one or more creatures.
Note that the spell is formulated to create the same number of arrows regardless of the number of targets selected. The last clause makes it clear that multiple arrows can be aimed at the same target, but even the first two sentences alone are enough to determine this.
As for your specific examples:
Neither Steel windfall Yet acid splashes can attack the same creature twice
The target clause for this spell is:
Pick up to five creatures you can see within range. Perform a melee attack on every target.
You first select the goals and then you can set them one Attack against any chosen target. If you select only a single target, you can only carry out a single attack. The same logic applies to acid splashes: When targeting a single creature, you are not targeting 2 creatures. Acid splashes every target only once.
Any other interpretation leads to absurd results
If you stipulated that effects like this could select the same target multiple times to stack multiple effects on that target, it would make the Hunter Ranger's whirlwind attack infinitely powerful:
You can use your action to perform a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.
Just choose to attack the same creature 1000 times. Any number is there, right? Better yet, choose each Creature within range 1000 times and instantly kill them all. This is clearly not a sensible decision. There are surely other ridiculous side cases like this if you allow multiple targets of the same creature this way.