How to create x amount of duplicates based on a row in the dataframe and change a single or multi variables from specific columns. The rows are then added to the end of the same dataframe.

  A B C D E F
0 1 1 0 1 1 0
1 2 2 1 1 1 0
2 2 2 1 1 1 0
3 2 2 1 1 1 0
4 1 1 0 1 1 0 <- Create 25 Duplicates of this row (4) and change variable C to 1
5 1 1 0 1 1 0
6 2 2 1 1 1 0
7 2 2 1 1 1 0
8 2 2 1 1 1 0 
9 1 1 0 1 1 0 

I repeat only 10 times to keep length of result reasonable.

#    Number of repeats |
#                      v
df.append(df.loc[[4] * 10].assign(C=1), ignore_index=True)A  B  C  D  E  F
0   1  1  0  1  1  0
1   2  2  1  1  1  0
2   2  2  1  1  1  0
3   2  2  1  1  1  0
4   1  1  0  1  1  0
5   1  1  0  1  1  0
6   2  2  1  1  1  0
7   2  2  1  1  1  0
8   2  2  1  1  1  0
9   1  1  0  1  1  0
10  1  1  1  1  1  0
11  1  1  1  1  1  0
12  1  1  1  1  1  0
13  1  1  1  1  1  0
14  1  1  1  1  1  0
15  1  1  1  1  1  0
16  1  1  1  1  1  0
17  1  1  1  1  1  0
18  1  1  1  1  1  0
19  1  1  1  1  1  0

Per comments, try:

df.append(df.loc[[4] * 10].assign(**{'C': 1}), ignore_index=True)