I have a high level Arcana Cleric/Wizard. Using wish to create a simulacrum, I can have a body double. Since I also took magic jar, my copy knows it, too.
I wanted to know what would happen if I ordered the simulacrum to possess me. Specifically, if I am attuned to the following magic items, will my body double inherit my attunement and thus gain the items' benefits?
Whether or not it is your Simulacrum double or another creature possessing you is irrelevant, as a Simulacrum is not you, but rather "an illusory duplicate" per the spell description. So you couldn't just hand over your items and have them already be attuned, or have a spell that targets self also target them.
So that leaves the more broad question of whether or not item attunement is inherited via Magic Jar possession. The spell says your stats (aside Intelligence, Wisdom and Charisma) are replaced by the creature's, and you retain your alignment and class features without gaining theirs, but it speaks nothing to attunement.
Reading the rules on attunement, we see that attuned items are attuned to a "creature", but as with anything related to Magic jar, the definition of creature becomes a bit vague here. Is it the body or the mind? The use of both "creature's body" and "creature's soul" in the text seems to imply that it's a bit of both.
As far as I can tell there is no defining answer given in any of the source books to clear this up, which means it'd be purely up to the DM's interpretation as to whether or not attunement transfers. It is worth mentioning however, that while not an official ruling, one of the co-creators of D&D 5e has offered their input on this exact topic, to which they ruled that attunement is to the mind and not the body.
In summary, RAW is silent on the matter, while RAI seems to be that they would not inherit the attuned items.
