perl, 25 bytes

Result

AC

Duration

850ms

Code [DL]

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

stdin

455 553
717 324
432 277
677 776
437 476
434 583
233 366
665 682
249 339
256 553
215 185
156 312
219 219
853 533
616 617
698 988
874 477
766 965
691 182
965 762
596 667
155 771
233 197
573 225
256 694
189 994
855 668
866 649
965 663
266 768
419 491
599 624

stdout

55277733355553684611653269566669

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fff004bede0 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x564361623d30 /* 7 vars */) = 0
12    execve("/usr/bin/perl", ["/usr/bin/perl", "/volume/CODE"], 0x564361623d40 /* 7 vars */ <unfinished ...>
11    +++ exited with 0 +++
12    <... execve resumed>)             = 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 +++