You certainly do stick to it

[shootingstar]

. . .
George: Gee, it's funny, Emily. From my window up there I can just see your head nights when you're doing your homework over in your room.
Emily: Why, Can you?
George: You certainly do stick to it, Emily. I don't see how you can sit still that long. I guess you like school.
Emily: Well, I always feel it's something you have to go through.
. . .
(Thornton Wilder, Our Town, Act I)

The words certainly do seem to double or tautologously emphasize the verb stick to in my opinion. What is the spirit and purpose of this usage? What does this sentence mean or express in this context?

 

[5jj]

The words simply strongly emphasise the main verb.

 

[Barque]

Using both those words might seem redundant, and he could also have said "You certainly stick to it" or "You do stick to it" but in this case the use of both words is idiomatic.

What does this sentence mean or express in this context?

Roughly, "You really work hard."