Doh! I'd included sbrk.o twice...

libc/sysdeps/linux/common/Makefile

@@ -27,7 +27,7 @@ include $(TOPDIR)Rules.mak
 CSRC=	waitpid.c kernel_version.c statfix.c getdnnm.c gethstnm.c \
 	mkfifo.c setegid.c wait.c errno.c getpagesize.c seteuid.c \
 	wait3.c setpgrp.c getdtablesize.c create_module.c ptrace.c \
-	cmsg_nxthdr.c open64.c statfix64.c statfs64.c sbrk.c
+	cmsg_nxthdr.c open64.c statfix64.c statfs64.c
 ifneq ($(strip $(EXCLUDE_BRK)),true)
 CSRC+=sbrk.c
 endif