An echo can happen usually in 2 scenarios.
Scenario 1: You are on screen with a guest and you hear an echo of your audio or guests audio.
This is typically caused by the speakers on one computer feeding back into the mic. Some computers are better at handling echo-cancelation than other but the best way to prevent this is to have all guests on screen wear headphones. If you still hear a faint echo after that, try lowering the volume of your headphones and it will go away.
If for some reason you're still not able to stop the echo, you can also click on the mic on your guest's video feed to mute them when they are not talking.
Scenario 2: You are watching a crowdcast as an attendee and hear the speakers audio twice.
This is caused by having the event open in more than one browser tab or window. Check to see if happened to have opened the crowdcast event multiple times by accident and close all other crowdcast pages.