Fix 'the the' typos across a whole document, or deduplicate a word list, one pass, order preserved.
Doubled words are among the most common typos in fast writing, 'the the', 'and and', and among the hardest to spot when proofreading, because the brain auto-corrects them. The adjacent mode removes only immediate repeats, which is the safe proofreading pass for prose: writing that intentionally repeats a word elsewhere is untouched. The all-duplicates mode is the list-cleaning pass: paste a keyword list, tag list or vocabulary set and every word survives once, in its original order and with its original capitalisation.
Punctuation handling decides whether 'end. End' counts as a repeat, with punctuation treated as spacing it does, which is what proofreading wants across sentence boundaries. Words are compared exactly otherwise, so 'run' and 'running' are never merged.
Will it damage normal prose? In adjacent mode, no legitimate construction is harmed except rare stylistic doubles ('had had', 'that that'), which are flagged by removal, read the changed spots. Keep a copy of the original for anything precious.
What counts as a word? A run of letters, numbers and apostrophes. Everything else is spacing or punctuation and is preserved in place.
Can it deduplicate lines instead? That is a different shape of data, the Remove Duplicate Lines tool on this site handles line-based lists.