Paste a list and keep each line once, in the order it first appeared.
Lines are compared after optional trimming, so apple and apple with a trailing space count as the same entry. Case sensitivity is off by default because most lists are meant to be case insensitive.
By default the first copy of each line survives and the rest go. You can keep the last copy instead, which is useful for logs where the newest entry matters. The show only duplicated option flips the tool around and returns just the lines that appeared more than once, which is a quick way to audit a list.
No. Lines keep the order of their first appearance. If you also want a sorted list, run the result through Sort Words Alphabetically.
Usually because of invisible differences such as a trailing space, a tab or different capitalization. Turn on trimming and turn off case sensitivity, and check for non-breaking spaces with the Whitespace Remover.
The tool runs in your browser, so the limit is your device memory. Lists of several hundred thousand lines work on a modern computer.