Avoiding duplicate messages being sent

Could you please help. I'm looking to add lists of emails as different groups
There's a good chance that the same email address may be imported each time, so the same address could appear in multiple groups.

It looks like there's an option called "Don't resend to people that had this newsletter sent to"
But I'm not sure if that does what I want - or something else, as it's not very clear what it means, and there doesn't seem to be much documentation about it!

Could you please advise what's the best way to achieve what I need to do?