Heavily related to this question... But I don't think it's a duplicate.
The sending spell states:
You send a short message of twenty-five words or less to a creature with which you are familiar.
And there is great debate about what constitutes "familiar". But what if you're familiar with someone deceiving you?
- You're very well acquainted with the mysterious shop keeper; or so you think. It's really 3 kobolds in a cloak!
- You've had drinks numerous times with the salty dwarf named Barney Rubble. However there is no Barney Rubble. It's just a disfigured tabaxi Warlock named Gem In Rolling Waters that only goes out using Mask of Many Faces.
- You pass by the same gentleman sitting in his rocking chair every day. But it's actually a doppelganger in one of their many shapes while the home owner is away.
- You're always doing odd jobs for the town constable, so you see him a few times a month to collect a reward. But little does anyone know, that he was assassinated last week. Instead, it's a minion, whom you've only met in passing once, posing as the constable as part of a larger plot.
In all these cases, the caster is familiar with "someone", but who the caster knows and the reality of that someone they think they know are very different.
So would the sending spell be directed to who you think you know, to the person beneath the disguise, or just fizzle out?