They are not duplicates because the recent question is about a wording interpretation problem for a specific spell
The older question asks:
In 5th Edition which spells, like Suggestion, are not noticeable, and which are noticeable by the target and/or nearby onlookers?
Are there differences if a target fails or succeeds on a save roll? [...]
The recent question asks:
The Charm Person spell description says:
When the spell ends, the creature knows it was charmed by you.
There is an ambiguity here — does this happen only if the target failed on the saving throw, or the result of the saving throw is irrelevant?
[...] According to the rules, it still "knows it was charmed". Does this happen if the target succeed on the saving throw?
On the one hand, the spell ends, so the target should "know it was charmed". On the other hand, one might argue the spell never took effect, so literally nothing happens when the saving throw is succeeded.
In order for the old question to cover the new one it would have to be asking answers to go through every single charming/persuading spell in the books and explain exactly which ones are noticeable, which ones are not, what happens on a successful save, and what happens on a failed save. This is the level of detail that the newer question is asking about since it is asking about the specific word choice in charm person in particular, not general things about these sorts of spells.
If the older question were asking for such a listing then, at least to me, it would be too broad. Thus, I assume that the older question is not asking for a detailed list of every charming/persuading spell in the game and explanations of what happens on successful/failed saves and thus it does not match the level of specificity that the newer question asks about and thus it is not a duplicate.
