Quick easy question.

[castorz]

I recorded a track but it only recorded from the left speaker, how can I duplicate it under?

Thanks a lot.

[steve]

Click on the name of the track and from the drop down menu select "Split Stereo Track"
Delete the silent track (click on the [X] in the top left corner of the track)
Ctrl+A (select All)
Ctrl+D (Duplicate)
Click on the name of the upper track and select "Make Stereo Track".

However you are probably asking the wrong question.
If you are recording a mono input (for example a microphone) then you should set the number of recording channels to "1 (mono)"
Edit menu > Preferences > Audio I/O

This will produce a single (mono) track that plays through both left and right speakers.

[castorz]

Thank you so much, that has done the trick. However I didn't find the Audio I/O in the Preferences menu.

[steve]

You have posted in the "Audacity 1.2.x" part of the forum, so I assumed that you were using Audacity 1.2.6.
Which version of Audacity are you using? (look in the "Help menu > About Audacity")