We've often seen La Nina episodes span multiple years, so having La Nina occurring in two consecutive winters isn't necessarily that unusual (for instance, 2008 / 2009 and 2011 /2012).  You can see historical La Ninas in the linked table (numbers colored blue).  Was is unusual is the warming in between the winters that approached El Nino.  This is something we hadn't seen before, but then again, we are only looking at about 65 years of data.

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI…