help mutex

[Didier FRAISSE]

i want to be sure that only one instance of my script is running at the same
time
i try this little script

#---------------------------------------------------------------------------
# -*- coding: cp1252 -*-

from win32event import CreateMutex
from win32event import ReleaseMutex
from win32api import GetLastError
from winerror import ERROR_ALREADY_EXISTS
from time import sleep

mutex = None

attendre = True
while attendre:
    mutex = CreateMutex ( None, 1, 'monmutexbienamoi' )
    if (GetLastError ( ) == ERROR_ALREADY_EXISTS):
        print 'another instance is running, i'm waiting'
        sleep(10)
    else:
        attendre = False
        print 'no instance, i'm running'

for i in range(10000):
    print 'i'm working...'

# release the mutex
print 'release mutex %s. another instance could run' %mutex
ReleaseMutex( mutex )
#--------------------------------------------------------------------------

but it doesn't work when i launch multiple instance of my script.
the first one is running without problem.
The other one are waiting and are never released.

where is the trap ??
Didier