Where duplicate lines actually come from
Duplicates rarely show up because someone typed the same line twice on purpose — they creep in from merging lists that came from different sources. An email list assembled from a sign-up form, a CRM export, and a manually maintained spreadsheet almost always ends up with overlapping entries, sometimes formatted slightly differently (extra spaces, different capitalization) in a way that makes them look distinct at a glance even though they represent the same person or the same value. The larger and more merged a list gets, the more this compounds, which is exactly why cleanup tends to matter most right before a list is actually used — sending a campaign, importing data, or publishing a final export.
The case-sensitivity trap
The most common reason "duplicate" entries survive a naive cleanup pass is inconsistent capitalization — Email@Site.com and email@site.com are functionally the same email address, but a strict, case-sensitive comparison treats them as two different lines and keeps both. The same problem shows up with a trailing space that's invisible when you're scanning a list by eye but makes two otherwise-identical lines register as different to any exact-match comparison. A dedicated dedupe tool that offers case-insensitive and whitespace-trimming options as toggles, rather than baking in one fixed comparison rule, catches both of these — which covers the large majority of real-world accidental duplicates.
Remove duplicates now
Why not just use a spreadsheet's remove-duplicates command
Spreadsheet software can absolutely remove duplicates, but it's a heavier process than the task usually calls for: import the list as a data range, find and run the remove-duplicates command (often buried a few menu levels deep), then export the cleaned result back out as plain text. A tool built specifically for deduplicating a pasted list skips all of that — paste the list, get the unique lines back immediately — and it sidesteps a real, if less obvious, risk of spreadsheet software silently reformatting your data along the way, like converting a long numeric ID into scientific notation without you noticing until later.
Getting a clean result
Turn on both case-insensitive matching and whitespace trimming as a default starting point unless you have a specific reason to keep strict, exact matching — most real duplicates in pasted lists come from inconsistent formatting, not truly distinct values, so leaving those options off tends to under-remove and leave near-duplicates behind. If the order of the final list doesn't matter to your use case, sorting the deduplicated output alphabetically makes it noticeably easier to scan and manually verify than leaving it in the original, somewhat arbitrary paste order the list happened to accumulate in.