There's a ton of repetition in this poem. The speaker repeats sounds and words like it's going out of style. The word "England" shows up three times, the words "green" and "pleasant" each pop up twice, and of course there's the anaphora of the third stanza ("Bring me my […] Bring me my […] Bring me my"). We also get alliteration in lines 4 ("pleasant pastures"), 9 ("Bring […] Bow […] burning), and 14 ("sword sleep"). On top of all that, there are at least two lines that rhyme in each stanza (and four lines that rhyme in the third). Okay, so we've clearly got a poem that loves to repeat itself. It's almost kind of like an echo chamber.
What's the point, you ask? It's like this. The speaker wonders if Jerusalem was once built in England long ago, and he wonders if Jesus was there too (even though he wants this to be the case, he's still not totally sure). Anyway, he also wants Jerusalem to be built in the future: "Nor shall my sword sleep in my hand: / Till we have built Jerusalem" (14-15). In other words, he wants the future to be just like the past. He wants history to repeat itself. In order to communicate that idea sonically, he repeats words and phrases throughout the poem. Past equals future.
Besides talking about this neat little trick, we should probably also say a quick word about how the speaker's tone shifts. When he contemplates the past, he sounds a little doubtful, but doubtful in the "I really want to believe this happened" kind of way. The way he keeps saying "And did" while not using an actual question mark makes that clear. At the same time, the way the "And dids" rack up prepares us for the speaker's sudden call to action in the third stanza: "Bring me my bow of burning gold: / Bring me my arrows of desire." This is his Braveheart moment, that's for sure, and he concludes the poem in that same state of mind. Even though the latter half of the poem echoes the earlier half, the speaker's tone has shifted from doubt to confidence and power.