In "The Wild Duck," truth and falsehoods clash in many ways and at many times.
This is foregrounded (and foreshadowed) by the first line of the play: " Is it true, what folks say, that they're -- very good friends, eh?" Jensen asks if something is true, but in doing so, he uses a euphemism. This lets us know that one of the clashes is going to be between the truth (as in, the facts) and the public face society puts on things.
On the level of plot and character, the truth is often a secret, and there are drives to expose it. Those who want to expose the truth think of ignorance or silence as a lie. Think of how Gregers wants Hjalmar to know the truth about Gina. That's a good example.
On the other hand, Hjalmar knows that fabrication functions as a "life-lie." Not facing the truth allows him to live, telling us truth is dangerous, even deadly.