#Mathematica, 72 bytes
""<>(t=Table)[s[[i]]~t~If[i~Mod~#2==1,#2,1],{i,Length[s=Characters@#]}]&
saved -2 bytes by listening to user202729
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Visit Stack Exchange#Mathematica, 72 bytes
""<>(t=Table)[s[[i]]~t~If[i~Mod~#2==1,#2,1],{i,Length[s=Characters@#]}]&
saved -2 bytes by listening to user202729