String_Agg is duplicating its values ...Please Help

Question

In this table i have a Parentname and Subname columns and in this case given Parentnames is the parent of given Subnames

 

 i was trying to do something like this:

 STRING_AGG(ISNULL(Distinct ch.Name, ' '), '; ') ParentName

but it is getting error..Any Solutions?I dont want to duplicate this  Parentname. i want it to be only one.