It happens that you have a hero in your team, and find the same hero in a dungeon. Screenshot :



http://cloud-2.steampowered.com/ugc/532875698423444240/97AAB97CB60F42A694F12963C5B911C16BDD1530/



Also, sometimes a hero dies, and you find him again. Sometimes you find him again in the same floor he died in. You can find dismissed heroes too, even if the text says they will be gone forever.