Rehash

Expansions should NOT be included as they largely can't be anything else (otherwise they wouldn't be expansions).

First occurences of versions/ports on other platforms should not count either.

The early part of Doom series (pre-Doom 3) is an exemplary case.

Another clear indicator is that players who are not intimately familiar with all games in a series may have trouble identifying one game in a series from one or more of the others, especially when even those who are very familiar with the series may have trouble identifying them (though there may be some signs they can check for, such as the double barrelled shotgun in Doom 2 that was not available in the original Doom).