Backreferences provide a convenient way to find repeating groups of characters. They can be thought of as shorthand for "match the same thing again".
A previous example presented the regular expression (?<char>\\w)\\k<char>, to find repeating adjacent characters such as the "ll" in the word "tall". The regular expression used the metacharacter \w to find any single "word character". The grouping construct (?<char> ) enclosed the metacharacter to force the regular expression engine to "remember" a sub-expression match (which in this case will be any single character) and save it under the name "char".
The backreference construct \k<char> caused the engine to compare the current character to the previously matched character stored under "char". The entire regular expression will successfully find a match wherever a single character is the same as the preceding character.
To find repeating whole words, the grouping sub-expression can be modified to search for any group of characters preceded by a space instead of simply searching for any single character. The sub-expression \w+ that matches any group of characters can be substituted for \w, and the metacharacter \s can be used to match a space preceding the character group. This yields the regular expression (?<char>\\s\\w+)\\k<char>. This regular expression will find any repeating words such as "the the" but it will also match similar words like "the" followed by "theory".
Adding the metacharacter \b after the repeat match will verify that the second match is on a word boundary. The resulting regular expression to find repeating whole words then becomes: (?<char>\\s\\w+)\\k<char>\\b