There are three main causes of echoes:
1) One or both users is not wearing headphones.
The echo can be removed by using headphones
2) One or both users is using iPhone headphones with a Mac
Due to the way that iPhone headphones are wired, this can cause an echo. Switching to another pair of headphones will solve this issue.
3) The lesson space is open in more than one window
This will cause an echo that can be solved by only having the lesson space open in one window.
If none of these situations apply to you:
Echoes can also be caused by an incorrectly configured Windows sound card - if you have not selected a default recording device under Windows, sometimes it will record from the headphones as well as the microphone, resulting in the other person hearing an echo. The solution is to make sure your microphone is selected as the default recording device.
If you hear a quiet echo of your own voice and the other user is using a headset, this is a likely cause. If you hear an immediate echo (with no delay) of your own voice, it generally means that your sound card is not configured correctly and you will need to make sure that your microphone is muted in the Windows playback volume settings.