Save-to-file code not quite working completely

[Steve]

I have a file containing specifications. My .py program can read and
manipulate the data but I cannot seem to get it to update the original file
properly.

The code is simple and works except that the original line remains in the
file after the new updated one has been added.

My code:
=======================================================

import fileinput
import sys

def ReplaceLine(file,searchExp,replaceExp):
    for line in fileinput.input(file, inplace=1):
       # if searchExp in line:
        line = line.replace(searchExp,replaceExp) #.rstrip()
        sys.stdout.write(line)
        
NewLine = "MSN Monitor Serial Number:  111111111-T4464                   ##
\n "
ReplaceLine("Specifications.txt","MSN", NewLine)
print()
PauseHere = input("Paused")
 =====================================
The text file:
1 MSN Monitor Serial Number:  111111111-T4464     ## 
  Monitor Serial Number:  88888888-T4464                  ## 
2 TLN TestStrip Lot Number:   45001 82624                  ##
3 SED Strip Expire Date:      2021-02-28                          ##
4 SEC Sensor Sequence Code:   68                                   ##
5 SCN Sensor Code Number:     F95                                  ##
6 SEN Sensor Serial Number:   0M000APJYWM             ##
7 SDE Sensor Date to Expire:  2020-12-31                       ##
8 SDN Sensor Day Number:      1                                       ##
9 DTD Data Time Date          Fri Aug 07, 2020 21:30       ##
=====================================
That second line shown was the original line for MSN. The replacement line
should have replaced the original line not just get added to the file.  It
should have been replaced. What is in line 2, should have "1 MSN" at the
beginning but that somehow disappeared.

So close, so simple...
How do I fix this?
Steve

P.S. I read to add ".rstrip()" but that messed things even more...  (-:


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