Hmmmmmmm.... Ive never programmed a hero wars map so i dont know about duelling triggers - however, I can tell you how to do hero selection. Its quite simple
First, go into object editor and make your "Choosing Unit" (This is usually a wisp). Remove all its abilities, such as build, harvest, detonate, etc, and you might want to change its name to "spirit" or something.
Then you want a small area of the map set aside for hero selection. Make it so you cant get from the hero selection area to the main part of the map, and vice versa. The easiest way is to make a circle of power (Units>Neutral Passive>Campaign) and put a hero in the middle of it. Now change the owner of the unit to neutral passive.
Lets call this hero "Bob" for now. Make a region around bob and call it "HeroBob". Now make another region where you want the player hero to appear. Call this "HeroSpawnBob". Now you have to make a trigger for it.
Events:
Unit Enters Region: "HeroBob"
Conditions:
[Unit-Type Comparison]
Unit-Type of [Entering Unit] is equal to "Choosing Unit"
Actions:
Unit - Remove [Entering Unit]
Unit - Create 1 Bob at "HeroSpawnBob" for player "Owner of [Entering Unit]".
If you want, you can add other stuff, but thats the general idea. If you want each hero to be chosen only once, you should add these two extra actions:
Trigger - Turn off [This Trigger]
Unit - Remove "Bob" (The neutral bob thats standing in the circle of power)
I hope that helps. If you have any other questions, just ask