On the roster
The list writes names as Dela Cruz, Princess Nicole M., so a middle name is usually all it takes to tell two learners apart at a glance. If one of them has no middle name recorded, add it: Learners → Edit → Edit details → Middle name, then Save learner.
The LRN column is the other reliable difference, and the private Notes field can hold anything else you need — it is only ever seen by you.
Both learners will sit next to each other in the list, since the order is by surname within gender.
Why a paste skipped the second one
Before anything is saved, every pasted row is checked against the roster you already have — by LRN, by email, and by name with capitalisation, punctuation and single-letter initials ignored. A row that matches is marked Already on the roster — skipped and its tick box is cleared, so pasting the same sheet twice is safe.
Two genuinely different learners with the same first and last name look identical to that check. The second one is skipped even though they should be added.
The fix: tick the row's box back on in the correction grid, fill in a middle name or an LRN so the two are distinguishable afterwards, then press Add 38 learners. Nothing else about the row changes.
Two rows in one paste
A duplicate inside the paste itself is flagged differently: Looks like row 7 — the same learner twice? That row stays ticked, because it is more often two real learners than one mistake. Untick it if it really is the same child pasted twice.
Two learners sharing an LRN
That one is always an error. The roster marks both numbers amber with the note Another learner has this LRN. One of them is a typo. Check both against your SF1 and correct the wrong one.