Dealing with duplicated data in a CSV file

[SamHobbs]

I am new to Python but I have been programming for half a century. The following uses techniques I might have used in COBOL but using Python of course. My data is a bit different from what you specify but I hope it is close enough. When you ask questions if you can provide a small amount of sample data as in the following then that helps those that want to help you. I am using:

key,f1,f2
1,data1,data2
2,data3,data4
3,data5,data6
4,data7,data8
5,data9,data10
5,data11,data12
6,data13,data14
7,data15,data16
8,data17,data18
8,data19,data20
9,data21,data22
9,data23,data24
10,data25,data26

The following:

import sys
import csv
with open('WithDuplicates.csv', newline='') as csvfile:
    dupsreader = csv.reader(csvfile, delimiter=',')
    datalist = list(dupsreader)
    n = len(list(datalist))
    print(f"{n} records")
    if n < 3:
        print("Not enough data")
        sys.exit()
    x = 2
    while x < n:
        if datalist[x][0] == datalist[x-1][0]:   # duplicate?
            print(f"{datalist[x-1][0]},{datalist[x-1][1]},{datalist[x-1][2]},{datalist[x][1]},{datalist[x][2]}")
            x = x + 2    # skip duplicate
        else:
            print(f"{datalist[x-1][0]},{datalist[x-1][1]},{datalist[x-1][2]}")
            x = x + 1

Produces:

14 records
1,data1,data2
2,data3,data4
3,data5,data6
4,data7,data8
5,data9,data10,data11,data12
6,data13,data14
7,data15,data16
8,data17,data18,data19,data20
9,data21,data22,data23,data24

That does not write the data out as a CSV but I hope that is close enough. I am not sure of exactly what you need as output. Note that that data has a header. Also note that that is (I believe) reading the entire file into memory. That should be okay (computers have ample resources compared to half a century ago) unless it is a really big file.