avenue33:
The trip fixed the issue.
Glad I was able to help. This issue should be fixed in the ReadBear duo hardware package so that others don't encounter the same problem you did.
avenue33:
All those non-Arduino platforms of to the rightarm-none-eabi-gcc. Why shouldn't Arduino?
Do you mean why doesn't the same issue occur with those packages? I think it's because those others don't have the conflicting definition of utoa(). I didn't find Simblee but all the rest except RedBear are using arm-none-eabi-gcc 4.8.3, RedBear is 4.9-2015-q3. This might be relevant: Conflicting type for first argument of utoa with stdlib.h declaration · Issue #4002 · arduino/Arduino · GitHub.
avenue33:
Issue #4593 has been closed despite not having been fixed.
No, it's still open. ElectorLabs closed it after we fixed their package but I requested that they reopen it because I think the IDE should be able to handle different packages using the same tool name and there was one other problem exposed in that issue.