I'm trying to append the repeated column in a dafaframe to new dataframe

Inuput

a	b	c	a	b	c	a	b	c
1	12	123	4	23	567	7	12	456
2	23	456	45	56	876	4	34	234
3	34	789	47	67	345	6	76	769

output

a	b	c
1	12	123
2	23	456
3	34	789
4	4	567
45	45	876
47	47	345
7	12	456
4	34	234
6	76	769

I'm trying to solve this in python

for i in range(0,len(supplier.columns),len(col_list)):
    dfnew = supplier.iloc[i : i + len(col_list)]

You can de-duplicate the columns with groupby.cumcount and compute a MultiIndex, then stack:

out = (df.set_axis(pd.MultiIndex
                     .from_arrays([df.columns,
                                   df.columns.to_series()
                                     .groupby(level=0).cumcount()]),
                   axis=1)
         .stack().sort_index(level=1)
      )

Output:

      a   b    c
0 0   1  12  123
1 0   2  23  456
2 0   3  34  789
0 1   4  23  567
1 1  45  56  876
2 1  47  67  345
0 2   7  12  456
1 2   4  34  234
2 2   6  76  769

Or using groupby and concat:

group = df.columns.to_series().groupby(level=0).cumcount()
out = pd.concat([g for k,g in df.groupby(group, axis=1)], axis=0)

Output:

    a   b    c
0   1  12  123
1   2  23  456
2   3  34  789
0   4  23  567
1  45  56  876
2  47  67  345
0   7  12  456
1   4  34  234
2   6  76  769