Repetition serves a variety of purposes throughout "The Ballad of the Landlord." The first instance is seen in line 1, of course, with the words "Landlord, landlord." This epizeuxis sets the stage for the poem's story, as it becomes clear to the reader that, despite its title, the ballad is actually being told from the perspective of the tenant. The repetition of the address also underscores the fact that the tenant—the speaker—must practically beg to get his landlord's attention, calling for him again and again.
This is further emphasized by the fact that the whole repetitive phrase is itself repeated in line 5. The fact that the tenant already tried to get the landlord to address the repairs to the property "way last week" makes it clear why the tenant is addressing the landlord with multiple calls for attention. The repetition adds a sense of urgency and pressure.
A similar use of repetition to convey urgency is seen in line 21. In this case, it's the landlord's voice, however, and he's calling for law enforcement: "Police! Police!" The epizeuxis reflects that, in the landlord's eyes, this is clearly a pressing matter.
There are other forms of repetition in the poem as well, such as the anaphora of the phrase "Ten Bucks" in lines 9, 10, and 11. In this case, repetition serves a different function in the poem. Instead of creating a sense of urgency, it serves to create a sense of incredulity–of sarcastic or dumbfounded doubt. The speaker uses a rhetorical question to express his disbelief, real or feigned, at the landlord's exploitive behavior:
Ten Bucks you say I owe you?
Ten Bucks you say is due?
It's not the amount of money that makes the speaker so incredulous but the fact that the landlord demands any money at all without bothering to do his own part and make the necessary repairs. By reiterating the landlord's ludicrous demand and reflecting it back at him with the repetition of "Ten Bucks," the speaker suggests just how incredible this request is. It's the landlord's duty to maintain the property so asking for cash for such essential repairs is simply criminal.