perl, 22 bytes

Result

AC

Duration

737ms

Code [DL]

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

stdin

626 228
456 552
789 961
667 679
574 472
477 119
938 562
597 367
833 295
181 559
544 414
321 218
922 225
144 558
818 551
535 556
774 727
797 678
153 865
855 859
881 911
155 558
824 263
957 351
977 492
513 316
136 661
361 729
611 712
626 527
476 617
833 311

stdout

25964777354424557775855573661163

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffe7f2b7e20 /* 7 vars */) = 0
12    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x55bb7dfc4d40 /* 7 vars */ <unfinished ...>
11    execve("/bin/cat", ["cat", "-"], 0x55bb7dfc4d30 /* 7 vars */) = 0
12    <... execve resumed>)             = 0
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
10    +++ exited with 0 +++