I have a dataframe that contains some rows that are equal with regard to all but one column, how could I merge the content of that one column they are not equal to each other

The question sounds complicated but here's what I have:

```
   col_a   col_b   col_c
   1       2       3,4,5
   1       2       3,6
   1       3       5

```

And what I want:

```
   col_a   col_b   col_c
   1       2       3,4,5,6
   1       3       5

```

Since row 1 and row 2 only differ in col\_c, I want to simply merge the two rows. But row 3 also differs in col\_a, therefore it must stay the same.

Any ideas how to approach this?