How to remove all the duplicated words on every line using Notepad++?

You can use a regular expression to remove consecutive duplicated words in a line, however I don't think it's possible to remove duplicated words which are not consecutive (e.g dangerous, hazardous, dangerous).

Use this regex in the replace window in Notepad++, and don't forget to select "Regular expression" as the Search Mode option below:

This regex will remove all consecutive duplicated words - whether it's 2 duplicated words or 10 duplicated words consecutively: \b(\w+)(?:,\s+\1\b)+.

The exact same no-commas regex would be: \b(\w+)(?:\s+\1\b)+ (might be useful for other users).

If you want a regex specifically for only two duplicated words (doubles), use this regex: (\b\w+\b)\W+\1.

Place this regex in the Replace with box to keep one occurrence of the word (otherwise all repeated words will be removed): ${1}.

These regular expressions will fix a situation like the one you described in your question as an example. The first regex will work for every number of duplicated words (e.g dangerous, dangerous, dangerous, dangerous, hazardous), while the second version will only work for two duplicated words (e.g dangerous, dangerous, hazardous).

Note: The regular expression will only apply to the format described in the question, meaning that formats like two words, two words, anotherword, two-words, two-words, anotherword, three words expression, three words expression, anotherword won't be changed because the regex won't apply to them.