Repetitions help to give the poem its sinister air, evoking both the circling language of old ballads and the speaker's obsessive hatred.
One of the creepier repetitions in the poem appears in the first stanza:
Who told my mother of my shame,
Who told my father of my dear?
Oh who but Maude, my sister Maude,
The speaker's diacope as she spits Maude's name drips with hatred: she knows exactly who betrayed her, and she's not going to forget in a hurry.
That gets even clearer when she repeats this repetition! Take a look at these lines from the last stanza:
But sister Maude, oh sister Maude,
Bide you with death and sin.
This echo brings the poem back to where it began; the speaker, it seems, is unlikely to move far from her ferocious hatred of Maude so long as Maude lives (which, if the speaker got what she wanted, might not be all that long).
Other repetitions connect this poem to the ballad tradition; ballads often use repetition for structure and emphasis. When the speaker describes her dead lover by declaring, "Cold he lies, cold as a stone," she both doubles down on his deathly chilliness and gives her line a swinging rhythm.
In the last two stanzas, meanwhile, the speaker uses the same turn of phrase twice as she develops a vengeful dream of the afterlife:
My father may sleep in Paradise,
My mother at Heaven-gate:
[...]
My father may wear a golden crown,
My mother a crown may win;
If my dear and I knocked at Heaven-gate
Perhaps they'd let us in:
This repetition stresses the speaker's idea that just about everyone might get into Heaven—except for the treacherous Maude, that is. It also creates a sing-song, nursery-rhyme sound that strikes an eerie contrast with the speaker's wish that her sister might be eternally damned for her betrayal.