Theory:

Repetition is one of the poetic devices used in poetry. Repetition is the technique of repeating the same words, phrases or lines a few times, to emphasize the idea - to make it more clear and significant; it is also used to create rhythm in poetry, to make it sound better.
Uses of repetition:
  1. Makes the idea clear.
  2. Emphasizes the idea in a stronger way.
  3. Makes the poetry sound rhythmic.
  4. Makes it easier to remember the lines.
  5. Helps in establishing the style and tone of the poetry.
Example:
1. A famous poem by Robert Frost - Stopping by Woods On a Snowy Evening uses the repetition device:
 
“The woods are lovely, dark, and deep,
But I have promises to keep,
 
And miles to go before I sleep,
And miles to go before I sleep.”
 
2. A famous poem by Walt WhitmanO Captain! My Captain! uses the repetition device:
 
O Captain! my Captain! rise up and hear the bells;
Rise up — for you the flag is flung — for you the bugle trills…”
Lines from the poem "Making Life Worth While" where repetition device is used:
Every soul that touches yours -
Be it the slightest contact -
Get there from some good;
Some little grace; one kindly thought;
One aspiration yet unfelt;
One bit of courage
For the darkening sky;
One gleam of faith
To brave the thickening ills of life;
One glimpse of brighter skies -
To make this life worthwhile
And heaven a surer heritage.
The word "one" has been repeated in the poem. The poet says we should get minimum one kindly thought, at least one aspiration, one bit of courage and we should take a glance at the brighter skies to make this life worthwhile.