I tried to use `Firmata.sendString(F("Encoder Warning: encoder is already attached. Operation cancelled."));` as described [here](http://playground.arduino.cc/Learning/Memory) but it is not supported (yet?) by the `sendString` method.

To avoid out-of-sram error, I propose to   : 
 * Comment this line so that nothing happen if user try to add multiple encoder on the same  index.
 * Catch this "error" on the client side (ex : on [firmatajs](https://github.com/jgautier/firmata) ).