With Arduino.CC IDE 1.6.8 and Arduino SAMD Boards 1.6.4 and RedBear Duo 0.2.7 installed, when compiling against the Arduino Zero, compilation fails and mentions the RedBear Duo as culprit.
In file included from /Users/ReiVilo/Library/Arduino15/packages/arduino/hardware/samd/1.6.4/cores/arduino/Arduino.h:40:0, from /var/folders/hz/wncgspfd5wxgzmly78yvzk_m0000gn/T/build150dbfda2222d62025a1934a4f9f0b3c.tmp/sketch/sketch_apr04a.ino.cpp:1: /Users/ReiVilo/Library/Arduino15/packages/arduino/hardware/samd/1.6.4/cores/arduino/itoa.h:29:65: error: conflicting declaration of C function 'char* utoa(long unsigned int, char*, int)' extern char* utoa( unsigned long value, char string, int radix ) ; ^ In file included from /Users/ReiVilo/Library/Arduino15/packages/RedBear/tools/arm-none-eabi-gcc/4.9-2015-q3/arm-none-eabi/include/stdlib.h:11:0, from /Users/ReiVilo/Library/Arduino15/packages/arduino/hardware/samd/1.6.4/cores/arduino/Arduino.h:25, from /var/folders/hz/wncgspfd5wxgzmly78yvzk_m0000gn/T/build150dbfda2222d62025a1934a4f9f0b3c.tmp/sketch/sketch_apr04a.ino.cpp:1: /Users/ReiVilo/Library/Arduino15/packages/RedBear/tools/arm-none-eabi-gcc/4.9-2015-q3/arm-none-eabi/include/stdlib.h:184:8: note: previous declaration 'char utoa(unsigned int, char*, int)' char * _EXFUN(utoa,(unsigned, char *, int)); ^
This issue has been posted at the RedBear Duo Forum also.