Requirements: To qualify to become a duelist, a character must fulfill all the following criteria.
Base Attack Bonus: +6. Skills: Acrobatics 2 ranks, Perform 2 ranks. Feats: Dodge, Mobility, Weapon Finesse.
Base Attack and Skill points are "things" that you must have a class to get (even racial Hit Dice count as a "class")! Feats on the other hand, can be a reward for leveling, you gain one feat at 1st, 3rd, 5th and on and on.... So, to get +6 base and 2 ranks on a skill you must had those from previous levels... but feats...
Let's say that Bob is a level 6 fighter, he has base +6, acrobatics & perform rank 2, dodge & mobility, but he still didn't get weapon finesse. at level 7 he gets a feat from leveling...
So, can Bob choose weapon finesse and right at the spot get Duelist, becoming Fighter 6/Duelist 1 or does he have to wait another level?