Okay, I got this game (Rio Grande edition) yesterday, haven't played it yet but have read the manual twice. Now, I strongly suspect that the answer to this question is "A" but I want to consult with you fine folks and make certain.
Page 6 of the rulebook reads: "When the last peace tile is placed on the game board, all the tribes are scored in order." Page 7 indicates that the game ends "as soon as the last peace tile is played on the board" (as one of three possible conditions). Then, "The active player finishes his turn and the tribes are scored as described above."
To me, this means one of two things:
A) If the game is concluded by the last peace tile being placed, then there is ONE scoring session according to the rules on pp. 6-7 and whoever has the most points is the winner (duh), OR
B) There are TWO scoring sessions in this scenario; one is triggered by the placing of the final peace tile, and the other is triggered by the simple fact that the game has concluded. Barring the use of action tiles, nothing happens in between the two sessions, so they will both yield the same results for all players. If the game finishes because of the last tribe marker from one tribe being placed or by someone reaching the topmost space of a tribe's influence track, then there is only one scoring session for the end of the game because there is still at least one peace tile left unplaced.