Учитывая блочный раздел, искать открытое LUKS устройство

Для записи, еще одна альтернатива с AWK без необходимости двойного чтения файла. Кажется, работает даже с несортированным файлом или даже с записями, найденными в файле только один раз.

$ awk '( a[$1] && (($2!=a[$1]) || ($1 in bad)) ) {bad[$1]++}; \
{a[$1]=$2;f1[FNR]=$1;f2[FNR]=$2}; \
END {for (i in f1) {if (!(f1[i] in bad)) print i,f1[i],f2[i]}}' ./tmp/file12


1 A T
2 A T
3 A T
12 C F
13 C F
21 E F

Где file12 выглядит так:

$ cat -n ./tmp/file12
 1  A T
 2  A T
 3  A T
 4  B T
 5  B T
 6  B F
 7  B F
 8  B T
 9  B F
10  B F
11  B F
12  C F
13  C F
14  D F
15  D T
16  D F
17  D F
18  D F
19  D F
20  D F
21  E F

Некоторые пояснения:

( a[$1] && (($2!=a[$1]) || ($1 in bad)) ) {bad[$1]++};
# The first time value $1 is checked,a[$1] will return null/0 since a[$1]
# has never been assigned. Thus a[$1] will be evaluated as false by
# condition check and further condition check and record bad marking will be skipped.
# The second time (second line) a[$1] will have a value and will be evaluated as true . 
# Then we check if $2 is equal to previous a[$1]=$2. If it is equal all ok.  
# Otherwise if current $2 <> previous a[$1] $2value, mark this record as bad.
# Finally there is a last check if record has been already been marked as bad.

{a[$1]=$2;f1[FNR]=$1;f2[FNR]=$2};     #some array delcaration

END {for (i in f1) {if (!(f1[i] in bad)) print i,f1[i],f2[i]}}' ./tmp/file12 
# Printing the lines of file that does not belong in bad boys.