Edited to add the solution I found:
In case anyone else is looking for an answer to this problem I went into Utilities>Audio MIDI on my Mac and under the 'Audio Devices' tab I selected SoundTap Recorder. That fixed the problem and there's no echo now so I'm very happy.
"I tried the free version of SoundTap but the playbacks all have an echo. I couldn't get help for this without purchasing the product and I've reluctantly done that but there is still an echo. I have SoundTap v2.34 Intel on my MacBook Pro running El Capitan. I followed the advice to select SoundTap in System Preferences&