"Echo" is full of repetition—fittingly enough, considering that an echo is itself a repetition!
The first stanza, for example, repeats the word "silence" (lines 1-2) and introduces the word "dream," which comes back as "dreams" in the third stanza. These repetitions create a hushed, ghostly atmosphere that hangs over the poem as a whole. They also highlight the fact that the beloved now exists only in silent dreams; the couple can no longer enjoy the normal interactions they once shared.
The first stanza also contains anaphora: the repetition of "Come" at the beginning of lines 1-3 and 5 (later echoed in lines 13 and 15). This anaphora creates an insistent, propulsive rhythm, pulling the reader into the poem and conveying the speaker's desperation to see their loved one.
The second stanza begins with diacope (the close repetition of "too") and epistrophe (the repetition of "sweet" at the end of successive phrases):
Oh dream how sweet, too sweet, too bitter sweet,
Through these repetitions, the speaker seems to revise or correct themselves, complicating their initial description of the dream. What first seemed "sweet" becomes "too sweet," then "too bitter sweet," as the speaker wakes up and realizes they've been dreaming. The reunion with their lover was only an illusion, a fantasy too good to be true.
Anaphora also appears in the speaker's description of "Paradise" (lines 9-10):
Where souls brimful of love abide and meet;
Where thirsting longing eyes
Watch the slow door [...]
The insistent repetition again conveys yearning: heaven is "Where" the speaker longs to go in order to be with their lover.
Line 12, in the same stanza, contains polyptoton:
That opening, letting in, lets out no more.
The speaker repeats the root word "let" in order to draw a simple contrast: the door of Paradise lets souls "in" but doesn't let them "out" again.
The final stanza also features various kinds of repetition. It echoes words like "come" and "dream"/"dreams" from earlier stanzas, keeping the poem's core ideas fresh in the reader's mind. Lines 13-14 contain another example of polyptoton ("live"/"life"), while line 16 contains diacope and parallelism: "Pulse for pulse, breath for breath."
Notice how most of these repeated words involve life and vitality. By emphasizing them, the speaker suggests that their life force is entwined with their lover's; in the lover's absence, they feel "cold in death." Similarly, the lover can live again only in the speaker's dreams, as though the speaker has the power to breathe life into them.
Finally, the last two lines of the poem contain diacope and epistrophe:
Speak low, lean low,
As long ago, my love, how long ago.
The repetition here adds both rhythm and emphasis. Notice that "low" and "ago" form an internal rhyme, making these last lines even more musical than the rest of the poem. Ultimately, all this repetition drives home the idea that the speaker is stuck in the past, desperately clinging to the "Echo" of their former happiness.