Cost: 1. XP: 2.

Choose a card in your play area or reveal a card in your hand. Search your deck for another copy of that card and draw it. Shuffle your deck.

"... all life is only a set of pictures in the brain, among which there is no difference betwixt those born of real things and those born of inward dreamings..." - H. P. Lovecraft, "The Silver Key"
Romana Kendelic
Dark Side of the Moon #205.
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For everyone's benefit, here is a list of the cards that most obviously synergies with Lucid Dreaming (as of Dream Eaters). Please mention any that I miss out in the comments:

Signature Cards:

Stack-able Hand Assets:

Disposable/Temporary Assets With Persistent Effects:

Most of the Myriad Cards:

Cards That Synergies With Themselves:

And that should about be it. Of course Lucid Dreaming has additional functionality outside of these cards; drawing the second copy of your 5xp sink or your assets with limited uses is always a good thing. And of course, not all of the cards presented are particularly powerful with Lucid Dreaming (e.g. Knife); this list it just a time saver for everyone who wants to know where Lucid Dreaming goes above and beyond a card draw.

Lucaxiom · 75
a) Yes you can trigger two copies of Survival Knife on the same attack as they are responses on two different cards. b) why would Hot Streak 4 and Expose Weakness 3 not make the list? — slothgodfather · 7
Expose Weakness (3) reduces the fight value of an enemy to zero. A second Expose Weakness (3) would also reduce the fight value to zero, AKA make no change at all. As for Hot Streak (4), I'll admit my reasoning is quite tenuous; essentially, you won't really need one copy of Hot Streak (4) to trigger another copy, as the cost of 3 resources is affordable. Hot Streak (2)'s 5 resource cost will generally need assistance from another economy card; it doesn't have to be another copy of itself, but it can be, and thus is included on this list — Lucaxiom · 75