Here is a way to do the job, this will replace all duplicate words evan if they are not contiguous:

• Ctrl+H
• Find what: (?:^|\G)(\b\w+\b),?(?=.*\1)
• Replace with: LEAVE EMPTY
• check Wrap around
• check Regular expression
• DO NOT CHECK . matches newline
• Replace all

Explanation:

(?:^|\G)    : non capture group, beginning of line or position of last match
(\b\w+\b)   : group 1, 1 or more word character (ie. [a-zA-Z0-9_]), surrounded by word boundaries
,?          : optional comma
(?=.*\1)    : positive lookahead, check if thhere is the same word (contained in group 1) somewhere after

Given an input like: dangerous,dangerous,hazardous,perilous,dangerous,dangerous,hazardous,perilous

We got:

dangerous,hazardous,perilous

Here is a way to do the job, this will replace all duplicate words even if they are not contiguous:

• Ctrl+H
• Find what: (?:^|\G)(\b\w+\b),?(?=.*\1)
• Replace with: LEAVE EMPTY
• check Wrap around
• check Regular expression
• DO NOT CHECK . matches newline
• Replace all

Explanation:

(?:^|\G)    : non capture group, beginning of line or position of last match
(\b\w+\b)   : group 1, 1 or more word character (ie. [a-zA-Z0-9_]), surrounded by word boundaries
,?          : optional comma
(?=.*\1)    : positive lookahead, check if thhere is the same word (contained in group 1) somewhere after

Given an input like: dangerous,dangerous,hazardous,perilous,dangerous,dangerous,hazardous,perilous

We got:

dangerous,hazardous,perilous