Yep, it's 1 every time. (just checked)

There's definitely not 2 checkboxes by the same id.
The view over which it open have something having the same Id as the name/id of these checkboxes.
Did not think it will be an issue, since I select by name, and nothing else share the same name.

Just for incase, I changed the checkbox names but same issue.

I realized at the moment my checkboxes get rendered (after name change) like this:
<input type="checkbox" name="selectedProvidersxxx" value="5" id="5">
(razor: <input type="checkbox" name="selectedProvidersxxx" value="@provider.ServiceProvider.Name" id="@provider.ServiceProvider.Id"/> )

Value / Id won't be part of the issue no?