= Solution
Collect distinct observed failure times as $a_m$, with $d_m$ failures and $r_m$ individuals in the <risk set> just before $a_m$. The grouped <Nelson–Aalen estimator> is
$$
\boxed{\widehat H_{\rm NA}(t)=\sum_{a_m\le t}\frac{d_m}{r_m}.}
$$
All failures recorded at one time use the same pre-event <risk set>. When failures and <censoring> share a recorded time, the usual convention includes those censored at that time in the <risk set> and processes failures before removing censored individuals. A known different ordering should instead be honored. If ties arise solely through rounding of a continuous-time process, an alternative is to resolve the unobserved ordering: $d_m$ successive failures without intervening <censoring> would contribute $\sum_{s=0}^{d_m-1}(r_m-s)^{-1}$. That is not the grouped jump $d_m/r_m$; the difference represents a choice about what the recorded ties mean.
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