Build error: duplicate declaration

[Riccardo]

When i try to build my project i get this error:

but i don't understand because i haven't duplicate declaration!
This is my code:

So you can see there isn't any duplicare declaration.
What is wrong?
Can you help me please?

[ABG]

This is a side effect of nb191.

Go into the Designer and rename any component that has a name like 'back' to include its type, like 'btnBack'.

[ChrisWard]

For the future, to take a snapshot of your blocks, use App Inventor's function - right mouse click in the Blocks workspace and select from the pop-up menu:

[Riccardo]

Ok for future i will do It. Howewer i solved renaming all components.
Thanks for all.