In case of several very long pages organized by tabs, is it better to repeat tabs at the bottom or to have a "Go to top" button?
Rather than have really long pages inside the tabs, fix the height of the tabs so that they occupy the full height of the screen and use a scroll viewer inside the tabs to display the content:
This way the tabs are always visible at the top of the screen where the user expects them to be. If there's enough height the scroll bar should be hidden.
You already took a look at how the component NavBar twitter? Seems like a good solution to your problem.
Here is the showcase of the components that created Twitter, scroll down the page and notice the menu. Twitter Bootstrap
If you develop using GWT, this is a fork of the twitter repository, with the components for GWT. GWT Bootstrap
This is very unusual and I'm not sure it's at all effective. Users might not understand whether these are the same tabs as above or some other tabs (internal maybe), and they might scroll to the top just to check this. On the other hand "Go to top" is kind of 90's.
Many websites use tabs with very long pages. Fixed tabs as had been suggested are one solution. Another could be to replicate the navigation, just not in the form of tabs:
Tabs at the bottom of the page are unusual, and therefore would likely be overlooked by the user (or cause confusion). Out of both options, I prefer the "Go to top" button.
If there is a logical flow between the tabs, consider having a link at the bottom to lead the user to the next tab. You could also have a few related links that lead the user to other tabs.
Another option is to have the tabs be static, so that they always appear on the user's screen. The downside to this is that they could take up valuable real estate that the user would value for reading purposes.