Info: (eintr) the-the

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 Appendix A The ‘the-the’ Function
 *********************************
 
 Sometimes when you you write text, you duplicate words—as with “you you”
 near the beginning of this sentence.  I find that most frequently, I
 duplicate “the”; hence, I call the function for detecting duplicated
 words, ‘the-the’.
 
    As a first step, you could use the following regular expression to
 search for duplicates:
 
      \\(\\w+[ \t\n]+\\)\\1
 
 This regexp matches one or more word-constituent characters followed by
 one or more spaces, tabs, or newlines.  However, it does not detect
 duplicated words on different lines, since the ending of the first word,
 the end of the line, is different from the ending of the second word, a
DONTPRINTYET  space.  (For more information about regular expressions, see 
 Regular Expression Searches Regexp Search, as well as *noteSyntax of
DONTPRINTYET DONTPRINTYET  space.  (For more information about regular expressions, see 
 Regular Expression Searches Regexp Search, as well as Syntax of

 Regular Expressions (emacs)Regexps, and *noteRegular Expressions:
DONTPRINTYET DONTPRINTYET  space.  (For more information about regular expressions, see 
 Regular Expression Searches Regexp Search, as well as Syntax of

 Regular Expressions (emacs)Regexps, and Regular Expressions

 (elisp)Regular Expressions.)
 
    You might try searching just for duplicated word-constituent
 characters but that does not work since the pattern detects doubles such
 as the two occurrences of “th” in “with the”.
 
    Another possible regexp searches for word-constituent characters
 followed by non-word-constituent characters, reduplicated.  Here, ‘\\w+’
 matches one or more word-constituent characters and ‘\\W*’ matches zero
 or more non-word-constituent characters.
 
      \\(\\(\\w+\\)\\W*\\)\\1
 
 Again, not useful.
 
    Here is the pattern that I use.  It is not perfect, but good enough.
 ‘\\b’ matches the empty string, provided it is at the beginning or end
 of a word; ‘[^@ \n\t]+’ matches one or more occurrences of any
 characters that are _not_ an @-sign, space, newline, or tab.
 
      \\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b
 
    One can write more complicated expressions, but I found that this
 expression is good enough, so I use it.
 
    Here is the ‘the-the’ function, as I include it in my ‘.emacs’ file,
 along with a handy global key binding:
 
      (defun the-the ()
        "Search forward for for a duplicated word."
        (interactive)
        (message "Searching for for duplicated words ...")
        (push-mark)
        ;; This regexp is not perfect
        ;; but is fairly good over all:
        (if (re-search-forward
             "\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move)
            (message "Found duplicated word.")
          (message "End of buffer")))
 
      ;; Bind 'the-the' to  C-c \
      (global-set-key "\C-c\\" 'the-the)
 
 
    Here is test text:
 
      one two two three four five
      five six seven
 
    You can substitute the other regular expressions shown above in the
 function definition and try each of them on this list.