I have put only and ONE only string like this in my image page: <?php echo hitcounter("image", false); ?> I am expecting the hitcounter to increase of 1 everytime I click on that image. The point is the FIRST time I see the image it actually increase of 1, then if I refresh the page or I decide to see the next image and then come back to the previous image then the hitcounter adds +2. Ok so I am a bit confused...

The hitcounter counts the visits/views (=hits) of that image page, not if someone clicks on the image itself.

Yeah I know. With "when click on that image" I meant to say "when I click on the image from the album page, and then I reach the image page" I can see the hicounter increases of 2 numbers.

But anyway the problem is still the same: let's say image A it's on page A where there's its hitcounter that states "10 views".
First time (after a long while) I visit the image A page it adds +1 (so I reach 11 view, so far it's ok)
but then
-if I refresh the page, the hitcounter adds +2 (from 11 it goes 13 views)
-or if I go back to the previous image page and then click next to the image A page, the hitcounter adds +2 (from 11 it goes 13 views)
- or if I go back to the album page and then click on the thumb of the same A image, I reach page A where the hitcounter adds +2 (from 11 it goes 13 views).

Hope my English was clear enough to understand.

I understood your problem. Sadly I can't reproduce it.