If two groups overlap, data will be shown twice in IP groups resource. Let me explain:
Let's say you have 2 IP groups - a bigger one called 4th floor (e.g. range 10.1.1.1-10.1.2.255) and overlapping subgroup HR office (e.g. range 10.1.1.25-10.1.1.30).
If there is a 100MB communication from HR office to exchange server - the traffic will counted to both HR office and 4th floor because otherwise you will get your sum of data for 4th floor wrong (e.g. 100MB less).
But that multiple reported data will only appear in IP group charts/reports, to report exact data transfer use total transferred bytes or protocol resource.