Pandas Intervals that Overlaps with the next row

Question

I have this problem to group overlapping intervals, the data is sorted, only need to find and group whether there are overlapping intervals between a row with the next one to it, not overlapping on all rows.

csv_file

ID  start       end
1   01-04-2011  01-04-2011
1a  01-04-2011  30-09-2011
2   01-01-2012  31-03-2012
3   01-04-2012  31-10-2012
4   01-11-2012  31-03-2013
6   01-04-2013  31-10-2013
6a  01-10-2013  31-03-2014
7   01-04-2014  31-10-2014
9   01-11-2014  31-03-2015
10  01-04-2015  31-05-2015
11  01-06-2015  31-10-2015
12  01-11-2015  31-03-2016
13  01-10-2016  31-03-2017
14  01-04-2017  30-09-2017

ID1 start and end are the same means that it has no end yet. I need to determine are ID1 and ID1a are overlapping, if not then are ID1a and ID3 overlapping? and so on. (ID is oversimplified)

I've searched everywhere and can't seem to solve this.

the expected result, the group will always consist of 2 intervals and they'll always be next to each other since the CSV is already sorted.

ID  start       end         overlap
1   01-04-2011  01-04-2011  Y_group1
1a  01-04-2011  30-09-2011  Y_group1

ID  start       end         overlap
6   01-04-2013  31-10-2013  Y_group2
6a  01-10-2013  31-03-2014  Y_group2

I found this

but I got the same error ValueError: need at least one array to concatenate

I found this solution but it returns so many True, maybe because of ID1?

intervals = df.apply(lambda row: pd.Interval(row['start'], row['end']), axis=1)
overlaps = [
    (i, j, x, y, x.overlaps(y)) 
    for ((i,x),(j,y))
    in itertools.product(enumerate(intervals), repeat=2)
]