Obosh, the Prey Peircer and Chandra's Ignition Bug

I cast Chandra's Ignition, Targeting Obosh, (My commander who was on the battlefield), however, the double damage only applied to the first instance of damage, not all instances.

I expected it to deal 6 damage, Obosh's Power is 3 damage then doubled by his effect, to all creatures and players, (one opponent had a large board of 5/5's) however it only dealt 3 damage after the first creature took damage.