Ah but he already had a bounty though it lacked a true picture. And its been revealed that one the bounties are active its stays active until death, capture, or 100 years after last time seen (Jango doesn't count as that was a gag, plus he wasn't a big shot and fought against other pirates to save a marine he befriended)
Sanji's first bounty had the face that resembled Duval (who's eyebrows don't looke like Sanji's) so they wouldn't think its the same as Vinsmoke Sanji. The pic that came in the newspaper that showed the Strawhats return showed the back of Sanji's head, again it lacked a true face shot.And its been said that Sanji may not be so original a name, esp when one considers that before they thought Sanji of the Strawhats crew came from East Blue as that's where he was first seen, and the Vinsmoke Sanji was from North Blue. So it could likely be they thought it was different people esp with the bad pics. Its only when they got a proper pic of his face that they realized its the Sanji from East Blue is Vinsmoke Sanji.
Getting the 'only alive' on it cancels out the 'Dead' part of the former bounty that read "Dead or Alive".
True that… my mistake. Thanks