One of my players wants to know if they can stack all the bonuses for one favored enemy instead of picking a new one, basically picking the same enemy type twice in addition to the extra +2 bonus. the description of the ranger's favored enemy is on page 47 of the 3.5 edition player's handbook. It doesn't say you can't do this, but it seems to imply that you have to choose a new one each time.
No, it has to be a new creature type from that table.
At 5th level and every five levels thereafter (10th, 15th, and 20th level), the ranger may select an additional favored enemy from those given on the table. — d20 SRD
You can't pick an "additional" one by picking the same one. To add a type, it has to be new.