The reason and meaning of repeating

This repetition acts much like an exclamation mark to insist on the truth of what has gone before.

It should not be emulated, unless you are aiming at a comic effect. Note that Eliza Dolittle comes to Prof. Higgins to lose her vernacular usage:

I want to be a lady in a flower shop stead of sellin at the corner of Tottenham Court Road. But they wont take me unless I can talk more genteel.

That gentility, as she learns in Act III, is not merely a matter of pronouncing words properly. She must also master the terms and phrases and syntax of "proper" English. One of the vernacular tricks she loses is just this insistent repetition: it disappears after Act II and does not return until Act V, at the precise moment when Eliza realizes her true strength and taunts Higgins by deliberately reverting for a single sentence to her old way of speech:

Thats done you, Enry Iggins, it az.

That sentence is the climax of the entire play, but the form of expression is ironic. I presume that you, like Eliza, want to employ "proper" English, not the street speech of 1913. If so, avoid this use.

The quotations above are from the text of Shaw's Pygmalion; I don't know whether they're preserved in the script of the musical and the movie.