Problem upgrading 4.8 to 5.1

OK I'm trying to upgrade from 4.8 (pre 4.9) to 5.1 (using the 5_1 tag 
with cvsup)

The problem I get is this:

cc -fpic -DPIC -O -pipe -DPTHREAD_KERNEL 
-I/usr/src/lib/libpthread/../libc/include 
-I/usr/src/lib/libpthread/thread 
-I/usr/src/lib/libpthread/../../include 
-I/usr/src/lib/libpthread/arch/i386/include 
-I/usr/src/lib/libpthread/sys 
-I/usr/src/lib/libpthread/../../libexec/rtld-elf -fno-builtin 
-D_LOCK_DEBUG -D_PTHREADS_INVARIANTS -Wall  -c 
/usr/src/lib/libpthread/sys/lock.c -o lock.So
building shared library libkse.so.1
thr_libc.So: In function `sigaction':
thr_libc.So(.text+0x54): multiple definition of `_sigaction'
thr_sigaction.So(.text+0x0): first defined here
thr_libc.So: In function `sigprocmask':
thr_libc.So(.text+0x34): multiple definition of `_sigprocmask'
thr_sigprocmask.So(.text+0x0): first defined here
*** Error code 1

Stop in /usr/src/lib/libpthread.
*** Error code 1

Stop in /usr/src/lib.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.


Is this a known problem and what can I do about it?

/John