calc, 103 bytes

Result

AC

Duration

1704ms

Code [DL]

=SUBSTITUTE(SUBSTITUTE(A1;REPT(A5;50);LEFT(A1;50));A3;REPT(A3;1+FIND(A4;A1)-MOD(FIND(A3;A1);51)))
K
T
 

stdin

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              K                                   

stdout

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              KKKKKK                                   

stderr

javaldx: Could not find a Java Runtime Environment!
Warning: failed to read path from javaldx
convert /tmp/code.csv -> /tmp/out/code.csv using filter : Text - txt - csv (StarCalc)

strace