A visitor can come in more than once, and he/she can click more than once, and it still can be valid and counted. Conversions actually happen on the 2nd or 3rd click (for PPC ads), so it would be ludicrous to discard repeat clicks and/or mark visitors invalid for repeat clicks.
The visitor figures in your statistics are actually Unique Visitors. It also excludes any visitors that we think may be invalid. It should be used as an estimate / suggestion on roughly how many visitors your domain name gets. We try to get this number as accurate as possible, but we still have a long way to go. There is actually 1-2 more invalid visitor ideas I have, and on top of that, we do want to discard 404 visitors like Parked - but only to a certain degree. We would discard visitors from people linking to images that no longer exist (404). But we would not discount 404s such as domainname.com/subfolder/folder/somepage.html. Because if you were to link to a non-existant image on your parked domain from some website, that would count as a visit - the idea in the future is to make sure that those requests do not count as a visit. I believe that is the whole concept of Parked's 404 visitor thing - correct me if I'm wrong.
How long the visitor stays in our "banned visitor" database all depends on how abusive the "invalid visitor" is. The more abusive the visitor is (the more invalid clicks, hundreds/thousands of useless impressions, etc), then the longer it will remain in our system.
If you visit your own domain names, chances are it will flag you quite quickly as an invalid visitor. We don't compare it to our member data, but we can tell by your behavior that you are not valid. For instance, does the average visitor visit 8 different domain names in 1 day from the same computer? No. Does the average visitor refresh a domain name 30 times? No - chances are that would be a member of ours that is viewing changes to his/her layout, etc. And sure, if you are a very slightly abusive visitor, you'll be in our database for maybe a few days, but if you come back again tomorrow - that term may be prolonged.
A unique visitor to us is based on the day of the visit. So it's almost like 24 hours, except that the same visitor on August 8th at 11 pm and August 9th at 1 am is counted to use as 2 unique visitors since they are on 2 different days. But that same visitor again at 9 pm August 9th is still counted as the same one from 1 am.
We do record, although do not archive or display your domain impressions, search impressions, and search unique visitors. I have thought about including the search unique visitors down the line. That would be once we start showing data for the separate revenue types (ZeroClick, PPC, and whatever else is to come (CPA

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