Fatal error: Cannot redeclare ControllerCommonColumnLeft

Hiya, All of a sudden localhost I'm getting a 500 on front-end, but admin is still working fine.

I've popped the error_reporting on via the index.php in the root:
and it's returning this ...

Fatal error: Cannot redeclare ControllerCommonColumnLeft::__construct() in /Volumes/Sites/mysitename/public-html/system/storage/modification/catalog/controller/common/column_left.php on line 96

I installed the Viethemes - Extra Positions mod, but that was quite a few days ago and I can't see what I've modified tonight, but I must have done something.

So you are using an ocmod modification that apparently has a bug in it so it is doubling the class setup for columnleft.
Disable any ocmods and hit the refresh button on the modifications page after each.. Then go try and see if the error goes away. Do this until you find the culprit and request assistance from the mod author.

Another thought is that you have loaded the same mod twice... once through the modifications area in admin, and also uploaded the ocmod file to the system folder which acts as a localized file-based location for ocmods. Be sure it is only in one of those places and not both.