Shell: What are lines which are repeated ?

$ cat file.txt
21421415
42657547
65765234
85685634
24246547
12336467
76545253
12336467
46565865
65464364
34668747
21421415
67475473
$ cat file.txt | sort | uniq -c | \
awk '{ if ($1 > 1) { print $2 } }'
12336467
21421415