Same Key is appearing twice in the output of a MapReduce program when Combiner is used - mapreduce

Question

I have written a MapReduce code to calculate the mean of a set of values for each key. It works fine when I use just mapper and a reducer. But when I introduce a combiner in between to reduce the load on reducer, the keys are getting repeated in the result. TIA. Code is given below.
mapper.py:
#!/usr/bin/env python
import sys
from datetime import datetime
for line in sys.stdin:
data = line.strip().split("\t")
if(len(data) < 5):
continue
date, time, store, item, cost, payment = data
print("{0}\t{1}".format(datetime.strptime(date, "%Y-%m-%d").weekday(),cost))
combiner.py:
#!/usr/bin/env python
import sys
from datetime import datetime
oldKey = None
salesList = ""
for line in sys.stdin:
data = line.rstrip().split('\t')
thisKey, thisSale = data
if(oldKey and oldKey != thisKey):
print("{0}\t{1}".format(oldKey,salesList))
salesList = ""
else:
if(oldKey == thisKey):
if(salesList != ""):
salesList = salesList + ',' + thisSale
else:
salesList = thisSale
oldKey = thisKey
if(oldKey):
salesList = salesList + ',' + thisSale
print("{0}\t{1}".format(oldKey,salesList))
salesList = ""
reducer.py
#!/usr/bin/env python
import sys
from datetime import datetime
oldKey = None
meanSales = None
salesTotal = 0
count = 0
for line in sys.stdin:
data = line.rstrip().split('\t')
thisKey, thisSale = data
thisSaleList = thisSale.split(',')
thisSaleListFloat = list(map(float, thisSaleList))
meanSales = sum(thisSaleListFloat)/len(thisSaleListFloat)
print("{0}\t{1}".format(thisKey, meanSales))
Output
0 249.91917747419384
0 250.09807318775844
1 249.87984898663836
1 249.59593170284487
2 249.95321425419965
2 249.75339205149234
3 249.54634982922747
3 250.19731461573994
4 250.3129656082639
4 250.13323419720658
5 250.13367036331366
5 250.03468060131152
6 250.207532163134
6 249.67593639719652