remove duplicates from csv file and export to csv and json simultaneously

rem-dups.py

import csv
import sys
import os.path
import json

def writeJSONFile(dataArray, csvFileOut):
    with open(csvFileOut, 'w', encoding = 'utf-8') as json_file_handler:
        json_file_handler.write(json.dumps(dataArray))

def removeDuplicates(inputCsvFile, outputCsvFile, outputJsonFile):
    with open(inputCsvFile, 'r') as infile, open(outputCsvFile, 'a') as outfile:    
        # this list will hold unique postIds,
        postIds = []
        jsonArray = []
        results = csv.DictReader(infile)
        writer = csv.DictWriter(outfile, results.fieldnames)
        writer.writeheader()

        for result in results:
            postId = result.get('post_id')
            if postId in postIds:
                continue
            jsonArray.append(result)
            writer.writerow(result)
            postIds.append(postId)
        writeJSONFile(jsonArray, outputJsonFile)

if os.path.isfile(sys.argv[1]):
    csvFileIn = sys.argv[1]
    csvFileOut = csvFileIn.split('.')[0] + '-nodup.csv'
    jsonFileOut = csvFileIn.split('.')[0] + '-nodup.json'
    if os.path.isfile(csvFileOut):
        print('ERR: output file already exists!')
    else:
        removeDuplicates(csvFileIn, csvFileOut, jsonFileOut)
else:
    print('ERR: input file not found!')