A double barreled question is one that is really two questions in one. See the example below:
Objective:
To know the experience level of participants who attended an editing workshop.
Very Poor:
Had you ever edited Wikipedia and Commons before this workshop?
This is actually asking two questions; whether participants edited Wikipedia or whether they edited Commons.
Poor:
Had you ever edited Wikipedia before this workshop?
Had you ever edited Commons before this workshop?
While some participants may have edited commons, most people typically participate in commons by uploading a photo.
Better:
(all three)
Have you ever edited Wikipedia?
Have you ever uploaded a photo to Wikimedia Commons?
Have you ever edited Wikimedia Commons?
These three questions asked separately better gets at editor's previous experience on Wikipedia and Commons. More questions could be used as well to get a sense of participants' level of experience.