Well, it matters a lot that it’s exactly the same as the poem’s first line. A title always gives a poet an important chance to draw our attention to a particular part of the poem. Here, Whitman uses it to underline the main image, that quiet spider. At first glance, maybe that just seems lazy – like Whitman stayed up late and couldn’t think of a title, so he just slapped the first line on there again and went to bed. But, the spider really matters. It’s a kind of a key to the meaning of more abstract parts about the soul. So, it makes sense that he really wants to burn that character into our brains.
It always helps to think about the other ways Whitman could title the poem. He could give away the whole plot, and call it "The Spider and the Soul." How would this change the feeling of reading the poem? When this poem was first published, it didn’t even have a title. It was just poem number three in a series called "Whispers of Heavenly Death." (For more on this, take a look at the original pages in the link below called: "Images of the Poem’s First Printing"). How might you react to the poem if you read it like that for the first time?