Most newsletter providers offer address management based on lists. You can imagine lists as card boxes in which you collect your business cards. With the old method, you simply put the business cards in a large card box. Since you are a very tidy and structured person, you also sort the addresses.
At some point, after the umpteenth networking event, you realize that you have several business cards from some contacts. But at which events did you meet again?
Next time you'll be smarter and create a separate box for each event. Now you'll know who you met at which event.
If you now want to write to bosnia and herzegovina telegram screening all contacts, you suddenly have a new problem. You have duplicates, i.e. one contact in several boxes. This means that if you write to everyone now, different contacts will receive your letter multiple times.
Not good.
Now you can do what is known as a duplicate check. If you have a manageable number of business cards in a box, this might be relatively easy to handle, and it can also be done easily in an Excel table.
The only problem is that list-based providers usually don't offer you such a duplicate comparison (although they certainly could). Their business model is based on the number of addresses in your lists.
Every address in every list counts towards the total. If you have "fans" who download all of your freebies, also sign up for your webinars and, "just to be on the safe side," have also signed up on your site using the newsletter form, you suddenly have the same address four, five or even more times in your lists.