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.

Sign Magick is a combo card, and one with rather strict limitations. You need to have two different Spell or Ritual assets in play, both with an => activation cost. And you need to have useful targets for those assets.

Would you like to combine Shrivelling and Rite of Seeking? Then you need an enemy and two clues on your location. Want to combine Sixth Sense and Rite of Seeking? Fine, but there better be at least 3 clues on your location for that to be worth a RoS charge, and it better be your last action this round (Since a special token + Rite of Seeking will end your turn early). What about earning cash from Alchemic Transmution while searching for clues with Sixth Sense? Yep, that can be effective but you need to include Alchemic Transmution in your deck - a card that's almost never worth it unless you can activate it for free.

In reality it's hard to line up three, often expensive assets in Arkam Horror, and then find enough useful situations to make Sign Magic worth your time. Especially since Sign Magic is quite useless on its own and since the combo falters as soon as one of your spell/ritual assets run out of charges.

I can see some use for this card, however. For example an Akachi deck with lots of spell assets. She has her own charge economy, so it's easier for her to find room for the necessary pieces and get them in play.

Will add some pieces of data I was looking for when entered this comment section and didn't find it fully. I will repeat something of what was already mentioned, trying to make my own review pretty comprehensive. Everything turned out to be in the rules, though, but who reads them before they need it? DISCLAIMER: I will be using some encounter cards from different cycles as examples, so if you care about that sort of spoilers, don't follow my links).

• What is "skill test from an ability"? I was sure that ability is only what is triggered, but I was wrong. Yes, there are triggered abilities (if a scenario card has any type of arrow, triggering which you initiate a skill test, you can commit this card, e.g. Bedeviled), but also: constant abilities (an effect that simply exists and doesn't have any specific point of initiation according to the rules. As I understand this card fits), forced abilities (something after word "Forced" that initiates a skilltest. In theory, bc I didn't find examples) and revelation abilities (skill test after word "Revelation", so, yes, basically half or even more cards of the encounter deck, like our favorite Rotting Remains or Crypt Chill). That's when I got true understanding what is this card about.
• What is "scenario card?" From rules: "Scenario cards include act cards, agenda cards, location cards, treachery cards, enemy cards, and scenario reference cards." and "Weaknesses with an encounter cardtype (such as enemies or treacheries) are considered to be player cards while they are in their bearer’s deck, and are considered to be encounter cards while they are being resolved, and once they have entered play". So you can find some clues while committing True Understanding to Searching for Izzie. And don't forget that abilities on weaknesses in threat area or attached to the location can be triggered by other investigators in the same location (yes, pure Jenny can spare her out-of-class card room for something more appealing while her seeker can watch her back). Above mentioned Bedeviled now can also be taken away from Finn with some additional use by your seeker. And, basically, any parley action with skill test can be assisted with True Understanding. This chatty Cathy Poltergeist! Always spills the beans.
• TL;DR the card is not that situational as one can find and, given you can commit it not only to your own skill tests, you are unlikely to find yourself in a situation when it's been uselessly sitting in your hand for half of a game without any chance to be triggered. It's much more easier to use it than, for instance, Evidence! while it is an actionless and 0-resource cost way to get a clue. The test, you need to pass, can also be easier than the location's shroud value.

A straightforward use for this is as a curse-heavy solo Rogue, at which point this simply turns half your curses into blesses. In theory, this averages the value of all curses to 0.

In practice, of course, you still have increased standard deviation, which is worse than not having the curses there at all... your worst-case scenario still means assuming all your curses come up cursey.

So perhaps the better way to look at it is as an automatic 50% saving-throw-vs-curses. Difficult to say if that's worth a 4xp one-of.

I have run this a couple of times with parallel Roland now, because one of his directives (Red Tape) makes Guidance fast! Now it may still not be the most optimal card, but now you don't lose an action playing it, which is good.