Find lines in file that contain duplicate characters

bash sorting duplicates character

grep '\(.\).*\1' file.txt

Matches any line that contains a character, any string, and then that same character, i.e. any line with duplicates.

bash sorting duplicates character

Perl:

perl -ne 'print if /(.)\1/' file.txt

awk:

awk -F "" '{for (i=1; i<NF; i++) if ($i == $(i+1)) {print; next}}' flie.txt

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