I can never keep straight in my head all the situations when an X value is and isn't preserved in Slay the Spire. One such situation is when a card with an X-Cost Reinforced Body triggers Echo Form. My guess is that the Value of X would be preserved when the second copy of the card is played.
However, I recently came across a situation where if the value of X wasn't preserved it would have meant losing the run. So before deciding what to play I tried searching online for confirmation of how Echo Form works with X-Cost cards, but I couldn't find an answer. Since experimenting in that situation may have resulted in losing the run, I elected not to let me X-Cost trigger Echo Form.
For next time, though, I'm hoping to get a definite answer to this question that I can easily reference when my memory fails me.
When activating X-Cost cards with Echo Form, is the value of X preserved?
Best Answer
When activating X-Cost cards with Echo Form, the value of X is preserved.
There are a couple notable cases where duplicating a card such as via Echo Form, Necronomicon, Double Tap, or Burst may not produce the expected results, though.
Also, perhaps more obviously, any card that evokes orbs will use the first orb, then the second orb, not the first orb twice.