A new type of Cauchy sequence

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I have very little idea about how to approach this question. It doesn't make any sense to me how this can be Cauchy. Any insight into it would be greatly appreciated.

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HINT: Suppose that $\langle a_n:n\in\Bbb N\rangle$ is Daudhy, and let $\epsilon>0$. There is a $T\in\Bbb N$ such that $d(a_j,a_k)<\frac{\epsilon}2$ whenever $T<j<2T<k$. Use the triangle inequality to show that $d(a_j,a_k)<\epsilon$ whenever $j,k>2T$ and conclude that the sequence is Cauchy.