Just submitted this bug to Corona (although I didn't receive the bug submission email confirmation...), but I like to share here on the forum so other developers can be aware of the bug.
Bug: the show function (phase 'did') of a scene is being called twice
How to reproduce:
Create 2 composer scene files ( let's say scene A & scene B ). On the Scene A show function, inside the "will" phase, call composer.gotoScene("sceneB").
The "show" function (phase 'did') of scene B will be called twice.
Why it is happening and how to overcome:
That bug happens when you call the gotoScene() from the 'will' phase. So, a way to overcome this bug is to move the gotoScene() do the 'did' phase.
Sample BUG code: https://www.dropbox.com/s/i3toht3wplxdo36/code.zip?dl=0