This can be caused by recording both system audio and another source such as a microphone or stereo mix simultaneously. The two tracks can get slightly out of sync and cause the echo described.
If you desire to only record the audio coming from your computer, select “Record system audio” and “Do not record microphone”.
If you are trying to record your voice alone, select the desired microphone and uncheck “Record system audio”.
You can often fix existing recordings by deleting one of the audio tracks on the timeline.