Does my data get uploaded anywhere?
No. Everything runs locally in your browser and nothing is sent to a server.
Is this free to use?
Yes. It is completely free, with no account, no signup, and no usage limits.
Does it work on a phone?
Yes. The page runs in any modern mobile browser, on iPhone and Android alike, with no app to install.
Why does my paragraph read like broken English afterwards?
Because every word is kept only on its first appearance across the entire text. Prose depends on repeating "the", "a", "of", and "is", and all of those repeats are deleted. That is expected behaviour, and it is why the tool suits word lists rather than sentences.
Can it remove only doubled words like "the the"?
No. It has no notion of adjacency, so it cannot target a repeated pair without also removing every later copy of that word. For a typo like that, use find and replace in your editor.
Are "cat" and "cat," treated as the same word?
No. The split is on whitespace, so the comma travels with the word and makes it a different token. Strip punctuation first with Remove Punctuation if you want the two to collapse into one.
Which capitalisation survives?
The first one you typed. Comparison is done in lowercase, but the word is emitted exactly as it first appeared, so a sentence-initial "Apple" will be the version that stays.
Are my line breaks preserved?
Mostly, but not reliably. A single line break between two surviving words stays, while a blank line becomes a space, and a line break next to a word that got deleted collapses into a space as well.