Double-jeopardy refers to the scientific fact that brands with higher penetration have higher loyalty than smaller brands and vice versa. In other towards, penetration and loyalty are firmly linked together - it's almost impossible to change one without the other changing too, Also, the variation in market share between brands is similar to the variation in penetration. The differences in loyalty are much smaller. For these reasons, it is widely agreed that in order to achieve significant long-term growth, a brand needs to focus on increasing its penetration.
The topic is discussed in this short video.