df = pd.DataFrame({"A":[1, 2, 3],
                   "B":[3,4,5]})

   A  B
0  1  3
1  2  4
2  3  5

df2 = pd.DataFrame({"A":[1, 2, 3, 2, 3, 3],
                    "B":[3,4,5, 4, 5, 5]})

   A  B
0  1  3
1  2  4
2  3  5
3  2  4
4  3  5
6  3  5