Echo happens when your microphone picks up sound from the Skype call and rebroadcasts it. There are two ways to avoid this:
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1.) Use a headset or headphones. Ensure that Skype's audio settings are configured to  use the headphones. 

2.) Or, if you do not want to wear headphones, use an external microphone, and turn on Echo Cancellation in Ecamm Live's Sound Levels window. (When using the built-in microphone, using Echo Cancellation can cause sound issues in Skype.)

If Echo Cancellation is not available, please check the article below for more details.