To add to Gail's reply, the optimizer parses the literal values in the IN predicate and extracts unique values.

Here's an example using your code:

DECLARE @T TABLE (

Column1 int,

Column2 int

)

INSERT INTO @T VALUES(1,2),(2,3),(3,4),(4,5),(5,6),(6,7),(7,8),(8,9)

SELECT *

FROM @T

WHERE Column2 IN (1, 2, 2, 3, 4, 5, 5, 5, 6, 6, 7, 8, 8, 8, 8, 8)

The scan predicate found in the execution plan is the following:

Predicate: [Column2]=(1) OR [Column2]=(2) OR [Column2]=(3) OR [Column2]=(4) OR [Column2]=(5) OR [Column2]=(6) OR [Column2]=(7) OR [Column2]=(8)

However, this is just an implementation detail you shouldn't care about.

EDIT: typo