I've found this excel add-in to buy that appears to do what I need, but I'd rather have code that's open to use as I wish. While a GUI is nice, it's not required.
In an attempt to make the question more clear, I'm adding some two sample "input" tables in tab delimited form, and the resulting output table:
SAMPLE_INPUT_TABLE_01
horse<tab>age<tab>body
a<tab>1<tab>A
b<tab>2<tab>B
b<tab>3<tab>C
c<tab>4<tab>D
e<tab>5<tab>E
f<tab>6<tab>F
SAMPLE_INPUT_TABLE_02
rider<tab>horse<tab>finish<tab>date
xxx<tab>a<tab>yy<tab>zz
xxx<tab>b<tab>yy<tab>zz
xxx<tab>b<tab>yy<tab>zz
xxx<tab>c<tab>yy<tab>zz
xxx<tab>e<tab>yy<tab>zz
xxx<tab>f<tab>yy<tab>zz
SAMPLE_OUTPUT_TABLE_03
rider<tab>horse<tab>finish<tab>date<tab>age<tab>body
xxx<tab>a<tab>yy<tab>zz<tab>1<tab>A
xxx<tab>b<tab>yy<tab>zz<tab>3<tab>C
xxx<tab>b<tab>yy<tab>zz<tab>3<tab>C
xxx<tab>c<tab>yy<tab>zz<tab>4<tab>D
xxx<tab>e<tab>yy<tab>zz<tab>5<tab>E
xxx<tab>f<tab>yy<tab>zz<tab>6<tab>F
If it matters, I'm using Office 2010 on Windows 7.