I'll copy/paste what I wrote from another thread, with added adjustments.
There are three products. FMA:M, a manga. FMA:1, an anime. And FMA:B, an anime.
FMA:M is simply called "Fullmetal Alchemist". It ran from August 2001 to June 2010. It was 108 chapters long. It was a manga and the orginal content from which the entire FMA universe is based. As par for manga, it had one author, Hiromu Arakawa. Unrelated but often confused fact: Arakawa is a female.
FMA:1, called "Fullmetal Alchemist", ran form October 2003 to October 2004. It was 51 episodes long. When production for FMA:1 began, Arakawa requested that it have a different ending from what she had originally planned. (But now that they're both out, the endings are remarkably similar.) FMA:1 has a lot of the same characters from FMA:M, a lot of the same plot ideas, and most of the same "magic". The plot, on the other hand, diverges significantly from FMA:M.
FMA:B, called "Fullmetal Alchemist: Brotherhood" (just "Fullmetal Alchemist" in Japan), is an anime based solely on the manga. It was 64 episodes long. It is produced by the same studio which made FMA:1, Bones. The plot, characters, scenes, and details from FMA:B are all exactly the same as from FMA:M. Although there are some minor differences in scenes (for example, scar wakes up in a bed instead of a tent), the main differences seem to be from story restructuring and shorting. Aside from details about the Ishvalan war, there was no major content differences between FMA:M and FMA:B.
To speak directly to your question, FMA:B is not a redo of FMA:1. If you have already seen FMA:1, I would suggest passively watching the first 13 episodes and then becoming more active after that. Although they are rushed and characters and events are not very different from FMA:1, there are many small details which will become important later on.
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