efficient data structure: column with repeating values

Is there some good data type out there that basically provides a simple
table, but with optimization for repeating values on one column?
Something like:

Data Table a b

...where it assumes that 'a' values will usually be unique, while 'b'
values will usually be repeated from a small set? (But not needing to be
fixed beforehand.)

Like in...

client | patron
---------------
Bob    | Tom
Sarah  | Tom
Dick   | Tom
George | Harry
Moe    | Harry

--

-- 
frigidcode.com

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