Why is network range overlapping on by default? Is there any benefit in turning it off?
With external IP's, we are not aware of a need for duplicate IP's. The main reason we have it on by default is that IP's should never repeat on the Internet, so it's a convenient way to catch an error by not allowing duplicates.
Therefore, the only reason duplicate IP's may be scanned is a user error: You are asking the system to scan Group A with IP's 1,2,3,4 and Group B with IP's 2,5,6,7 and do not realize "2" is in both groups. It's up to you at this point if you would like to stop and get notification that you have made a mistake, or just go ahead and scan all 8 IP's, with one of them repeating.
It's worth to note that from our perspective, duplicate IP's do not matter, we can scan the same IP in different scans and only have this feature for our customer's convenience. If in your case that complicates matters, you can go ahead and turn this option off. It's basically a convenience feature, so if it does not not help you, there's no need to have it on.