MS Excel Formula issue

I have data in excel in below mentioned format

Column1-> Column2
NI-Summer-2-> parent
NI-548 -> underneath are child
NI-549
NI-550

NI-Summer-1 -> parent
NI-421 -> underneath are child
NI-422

What i require is to display values in third column3 in below mentioned format

Column1 -> Column2 -> Column3
NI-Summer-2 -> parent -> NI-Summer-2,NI-548,NI-549,NI-550
NI-548 -> underneath are child
NI-549
NI-550

NI-Summer-1-> parent -> NI-421,NI-422
NI-421-> underneath are child
NI-422->


Actually condition is that value below each parent should be comma speared in next column of parent.

Please help to resolve this.

Thanks in advance.