multiprocessing gives wrong number of outputs, repeats outside scope - python-3.x

Question

When I use multiprocessing.pool.starmap it repeats the process outside the part I define as the process.
I am confused as to why.
MRA:
from multiprocessing import Pool
def countAdd( first, second, third ):
added = str( first + second + third )
with open( "test.txt", 'a' ) as f:
f.write( "this is string: " + added + '\n')
testlist = [ 1, 2, 3, 4, 5, 6, ]
testlist2 = [ 7, 8, 9, 10, 11, 12 ]
testlist3 = [ 13, 14, 15, 16, 17, 18 ]
arglist = [ (testlist[ test ], testlist2[ test ], testlist3[ test ]) for test in range( 0, len( testlist ) ) ]
print( arglist )
print( len( testlist ) )
if __name__ == '__main__':
with Pool( 3 ) as pool:
pool.starmap( countAdd, arglist )
Output:
[(1, 7, 13), (2, 8, 14), (3, 9, 15), (4, 10, 16), (5, 11, 17), (6, 12, 18)]
6
[(1, 7, 13), (2, 8, 14), (3, 9, 15), (4, 10, 16), (5, 11, 17), (6, 12, 18)]
6
[(1, 7, 13), (2, 8, 14), (3, 9, 15), (4, 10, 16), (5, 11, 17), (6, 12, 18)]
6
[(1, 7, 13), (2, 8, 14), (3, 9, 15), (4, 10, 16), (5, 11, 17), (6, 12, 18)]
6
file:
this is string: 21
this is string: 24
this is string: 27
this is string: 33
this is string: 36
So I'm really confused. Why are the print statements also printed repeatedly? But only 4 times?!
Why does the file only contain 5 times the output of the function and not 6 times, eventhough the arglist contains 6 tuples?
I searched around, found a similar probleme but isn't it.
Python Multiprocessing data output wrong
Thanks for taking the time to respond.
EDIT:
Based on the comments by MisterMiyagi I changed my code to the following, but there are still problems:
from multiprocessing import Pool
def countAdd( first, second, third ):
added = str( first + second + third )
with open( "test.txt", 'a' ) as f:
f.write( "this is string: " + added + '\n')
if __name__ == '__main__':
testlist = [ 1, 2, 3, 4, 5, 6, ]
testlist2 = [ 7, 8, 9, 10, 11, 12 ]
testlist3 = [ 13, 14, 15, 16, 17, 18 ]
arglist = [ (testlist[ test ], testlist2[ test ], testlist3[ test ]) for test in range( 0, len( testlist ) ) ]
print( arglist )
print( len( arglist ) )
with Pool( 3 ) as pool:
pool.starmap( countAdd, arglist )
print( "done")
I repeatedly ran the code. Sometimes I get the six lines of output but othertimes I don't. I ran the code 10 times and I got 54 lines of text in the output file. What is going on here?