I'm a little confused about this fender tag - although I've decoded all the codes, it seems to have an extra code (or duplicated code) where it shouldn't on the second line from the bottom:

M33 R11 V1X END
A04 C55 G33 H51 J25 M21
R6 R6 P6X X9 A11 137816
E31 D31 LP23 D9E XXXXXX

The R6 code for red - unless my mind was playing tricks on me and I'm imagining things - was repeated. Is this possible?

Not only possible, it was the way a tag was coded at some plants. As you can see, that's how the LA plant did it for 1969.

R6 R6 just means the top and bottom of the car were painted red as opposed to a two tone car that could code W1 R6 (White painted top and red lower)

Thanks for the quick answer - honestly, that was the first thing I thought when I saw the tag, since the "R6" code was easily recognizable. Unfortunately, when I got home and started plugging the rest of the tag into several of the decoders out there, there was no space for the second code and I started wondering if something was wrong... good to know this is just variation from plant to plant.