Each stanza of "The Road Not Taken" follows this rhyme scheme: abaab. The poem is a metaphor about choices in life. The speaker notes that both roads looked "really about the same," even though he tries to convince himself that he took the less traveled road. The implication is not that he took the less traveled road. In the last stanza, the speaker, later in his life, notes that he will say he took the road less traveled. But he says this with a "sigh" and this indicates that he's only saying he took the less traveled road. The sigh also indicates that he will wonder where the other road/choice might have led him.
In the last stanza, to keep the abaab rhyme scheme, Frost ended the middle line with "I" and "I" also begins the next line. This might have been a matter of idiosyncrasy and/or keeping to the rhyme scheme.
Two roads diverged in a wood, and I--
I took the one less traveled by,
And it has made all the difference.
But, the repetition of "I" could be a parallel to the two roads/choices the speaker's metaphor addresses. The repetition also shifts the emphasis from roads to the speaker himself. This shift is notable because this poem is only metaphorically about roads; it is about personal choices.