snobol, 105 bytes

Result

AC

Duration

744ms

Code [DL]

	T = TRIM(INPUT)
F	OUTPUT = T
	DIFFER('',K = TRIM(INPUT))	:F(F)
	OUTPUT = K DUPL(1,SIZE(T) - SIZE(K))
END

stdin

                             T                    
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
        K                                         

stdout

                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
                             T
        K111111111111111111111

stderr

strace