calc, 106 bytes

Result

WA

Duration

1806ms

Code [DL]

=REPT(LEFT(A1;51);B2/51)&REPT(C3;MOD(C2;51))&REPT(B3;MOD(FIND(A3;A1)-C2;51)),=LEN(A1),=FIND(B3;A1)-1
T,K, 

stdin

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             K                                    

stdout

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             KKKKKKKKKKKKK

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