Not much of a weakness, to be honest, although at least it messes with Amanda's mechanics. Looking at the two elements, the effect and the discard condition, we get:

The effect: If this is in Amanda's hand at the beginning of the Investigator Phase, she must put it underneath her, making her a poorer Preston. elkeinkrad's suggestions of how to deal with it are pretty solid -- commit it to a test (yours or someone else's) before the beginning of Amanda's turn or overwrite it with Obscure Studies. You can also just power through, doing testless actions or throwing in more icons than usual. There are a few cards that might let you find and discard this, but usually Amanda would prefer to get rid of her Basic Weakness, since it's likely to be harsher than her signature. Once in a while, this weakness will catch you in a very bad place, but, most scenarios, it will be the lightest of speed bumps.

The discard condition: Discard it at the beginning of Amanda's next turn or overwrite it with her signature.

All in all, this is a way below average signature weakness.

This card is super fun for Dexter, who can remove it from play without paying any of the negative effects. And most Dexter decks are overloaded with assets, so not committing cards during your turn doesn’t seem like a huge problem. At 4XP, it’s expensive, but slotless plus-one to every stat can be useful against diverse encounter decks like Innsmouth.

I'm loving this upgrade on Survival Knife (2), which allows you to fight and deal damage as a during the Enemy Phase BEFORE the enemy can damage you. That attack with this upgrade becomes ( + +2) and deals 3 damage (or more with Vicious Blow or Vicious Blow (2)).

Use your last action to move into a location with an enemy (or one location away from an enemy with Hunter) and deal at least 3 damage to that enemy when it attacks, which may kill it before it gets to damage you.

The Good, the Meh, and the Ugly:

The Good: A dollar off all items? Fantastic. It's doesn't even exhaust. Between this and Bob's free play-item action, you've got a mini Ever Vigilant forever.

The Meh: The second bit seems neat; in practice it hits the same barriers as Teamwork as you mess with other deck's plans for their limited Slots. Slotless items sidestep this, so here's hoping this cycle includes more things like Pills, Smokes, Booze... More Booze... uh...

...wow. Bob may be more of a bootlegger than an actual bootlegger.

The Ugly: SO SWINGY. Bob REALLY wants this in his opening hand. Get it anywhere from mid-game on, and you're probably just committing it for an investigate. And to my knowledge, there's literally no tutor that'll help. It feels wasteful to ignore, but risky to count on, sort of like Wendy's Amulet which you can at least find in a Backpack.

(EDIT: Building a Bob deck and realized that L3 Lucky Cigarette Case is an excellent tutoring option available to him, both for this and more or less anything else.)

+++

So, tl;dr... make friends with your local seeker!

Just here to say that I find the duplicate cards in the investigator packs rather nice, for multiplayer deckbuilding with a single physical collection. Most of these cards are considered essential for their class and you almost never want to go without two copies. Vicious Blow especially is one that you always want if you plan on doing combat, but if your friend is building a main fighter then you're stuck with either sharing and thereby making her deck weaker, going without, or buying a couple more core sets (at which point you'd be better off proxying, which I personally try to avoid). In the past my group has always just tried to pick investigators whose card pools don't overlap so nobody has to share or make suboptimal compromises. I still think that can be a fun limitation, but it's nice to have a bit more freedom through these extras.