I can reproduce it most often when I have uncommitted changes in an unrelated file, say board/cr50/board.c. Then run

  cd src/platform/ec
  \rm -rf build
  make buildall -j33

I get this:

  build/nucleo-f072rb/RW/chip/stm32/i2c-stm32f0.o: In function `i2c_init':
  /home/wfrichar/trunk/src/platform/ec/chip/stm32/i2c-stm32f0.c:606: undefined reference to `i2c_ports_used'
  /home/wfrichar/trunk/src/platform/ec/chip/stm32/i2c-stm32f0.c:606: undefined reference to `i2c_ports'
  build/nucleo-f072rb/RW/common/i2c.o: In function `get_i2c_port':
  /home/wfrichar/trunk/src/platform/ec/common/i2c.c:50: undefined reference to `i2c_ports_used'
  /home/wfrichar/trunk/src/platform/ec/common/i2c.c:50: undefined reference to `i2c_ports'
  build/nucleo-f072rb/RW/common/i2c.o: In function `command_scan':
  /home/wfrichar/trunk/src/platform/ec/common/i2c.c:786: undefined reference to `i2c_ports_used'
  /home/wfrichar/trunk/src/platform/ec/common/i2c.c:786: undefined reference to `i2c_ports'

I think it's because of cts/build.mk:

  ifeq "$(CTS_MODULE)" "gpio"
  CFLAGS_CTS+=-DCTS_MODULE_GPIO
  endif

  ifeq "$(CTS_MODULE)" "i2c"
  CFLAGS_CTS+=-DCTS_MODULE_I2C
  CONFIG_I2C=y

​Because if I capture what that does, I see these invocations in parallel:

  ​make build_cts
  make CTS_MODULE=meta BOARD=nucleo-f072rb
  make CTS_MODULE=interrupt BOARD=nucleo-f072rb
  make CTS_MODULE=timer BOARD=nucleo-f072rb
  make CTS_MODULE=gpio BOARD=nucleo-f072rb
  make CTS_MODULE=mutex BOARD=nucleo-f072rb
  make CTS_MODULE=i2c BOARD=nucleo-f072rb

​I'm guessing that having simultaneous builds which both #define and don't #define CTS_MODULE_I2C are fighting over how board.o is compiled.

I'm surprised you don't have a lot more of these collisions.