How do I find duplicate lines in multiple files within folders

when i want to find duplicate lines between two files i use this command comm -12 <(sort file1.txt) <(sort file2.txt) or sort file1.txt file2.txt | awk 'dup[$0]++ == 1' But, how do I find duplicate lines in multiple files within folders. example: mainfolder folder1 file1-1.txt file1-2.txt etc folder2 file2-1.txt file2-2.txt etc and that the result in terminal is displayed by file (that is, the lines repeated in all files but specify which file is the one that contains it) to know the origin of the problem. PD: I tried this command and it didn't work for me file_expr="*.txt"; sort $file_expr | sed 's/^\s*//; s/\s*$//; /^\s*$/d' | uniq -d | while read dup_line; do grep -Hn "^\s*$dup_line\s*$" $file_expr; done| sort -t: -k3 -k1,2 | awk -F: '{ file=$1; line=$2; $1=$2=""; gsub(/(^[ \t]+)|([ \t]+$)/,"",$0); if (prev != "" && prev != $0) printf ("\n"); printf ("\033[0;33m%s (line %s)\033[0m: %s\n", file, line, $0); prev=$0; }'