Hello @Kulkul, thank you for your answer I appreciate it but I published the question before I finished typing (Sorry about that, I'm new to the Forum) !
What I actually want dynamo to do is :
1-Group ducts by System type and then...
2- Group by Size.
3-Sum the lengths by Size.
For example :
400x200 : 12m
600x300 : 15m
It seems like I can either Group by System Type or by Size but not both.
Here's a snap of the humble result I got :