The following is from the specfication for Glyf tables in Apple's OTF file specification. http://developer.apple.com/fonts/TTRefMan/RM06/Chap6glyf.html
"If the x-short Vector bit is not set, and this bit is set, then the current x-coordinate is the same as the previous x-coordinate."
Is is not clear whether they mean the *actual* x coordinate or the *relative* x coordinate. What is actually stored in the table is relative offsets from the previously listed coordinate, so if the above statement refers to relative coordinates it would mean if you have a string of x coordinates "3,3,3,3,...." (indicating a constant angle for some stretch) that just one "3" would be stored for that stretch. OTOH if the above spec means *actual* x coordinates, it would mean that only in the case of a series of x's indicating a vertical line (i.e. no change in actual x coordinate) would all those x coordinates be collapsed to 1.
So I can't tell from the spec which they're referring to. And you can't tell from looking at most ttf files because the above spec only applies when x coordinates are 2 bytes as opposed to 1, which will be in an extreme minority as most x offsets form the previous x will be small.
So an obscure question maybe, unless you have to parse TTF/OTF files directly (as in an application). I wouldn't blame you guys if no one here knows this, but if knowledge of the TTF/OTF spec cannot be found here, then I don't know where.