Do conditions stack in Frosthaven?
No. A figure can only have one copy of each condition, so a second Poison or Wound changes nothing except the timer.
Two sources of Poison on the same monster are still one Poison. Each condition is either on the figure or not: a figure cannot hold multiple instances of the same condition.
If a figure that already has a condition gains it again, the condition's duration resets instead of stacking. That matters for timing — re-Poisoning a monster does not double the damage, it only restarts the clock.
Different conditions do stack with each other. A figure can be Poisoned and Wounded and Muddled at the same time; each one is tracked separately.
“A figure cannot have multiple instances of the same condition; however, if a figure gains a condition they already have, the effect's duration resets.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.