PHILOLOGOS explains the origins of the various words for "Rabbi" (the Forward). But in places he seems a bit confused on whether the words have one or two Bs. The Hebrew root is a geminate (root consonant II=III): RBB, so forms that he gives with a single B should actually have two Bs when the final root consonant(s) are followed by a vowel: "rabbi," "rabban," "rabbani." Forms like "rav"/"rab" which don't have a vowel following the final root consonant(s) just have one B/V.

Sorry you asked?