perl, 23 bytes

Result

WA

Duration

898ms

Code [DL]

/.\K(.)\1/,print$1for<>

stdin

277 179
823 645
234 346
866 168
968 886
993 932
867 487
469 965
397 793
911 195
725 571
755 557
756 366
755 375
977 754
963 641
618 315
524 264
276 227
863 662
522 992
499 525
841 218
553 384
115 718
617 969
244 665
624 227
594 446
932 776
955 553
744 974

stdout

77768888811565777726299999424754

stderr

strace

16    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffc15ab7920 /* 7 vars */) = 0
18    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x556878baed40 /* 7 vars */ <unfinished ...>
17    execve("/bin/cat", ["cat", "-"], 0x556878baed30 /* 7 vars */) = 0
18    <... execve resumed>)             = 0
17    +++ exited with 0 +++
16    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=17, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
18    +++ exited with 0 +++
16    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=18, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
16    +++ exited with 0 +++