In Casino Royale (2006), Felix Leiter introduces himself to Bond when he goes after Le Chiffre. However, in the previous Bond movies such as The Living Daylights (1987), Bond already met Felix Leiter. So, the question is, Why does Felix Leiter introduce himself to Bond when they have already met?
The 2006 film "Casino Royale" marks a reboot of the series and not a continuation of the previous films
Casino Royale reboots the series, establishing a new timeline and narrative framework not meant to precede or succeed any previous Bond film
In the context of Bond chronology "Casino Royale" occurs before "The Living Daylights. The novel "Casino Royale" marks the first of the James Bond series having been published 13 April 1953 while "The Living Daylights" is based on a short story from the book "Octopussy and The Living Daylights" which was published 23 June 1966.