C (gcc), 200 bytes

Thanks to @Kevin Cruijssen and @Laikoni for lots of saved bytes!

f(r,n,a){char*t=r,c[222]={!1};for(a=!1;*t;)++c[*t++];for(n=-1;222/(++n+1);c[n]&&c[n]-a&&(a=-1))!a&&c[n]&&(a=c[n]);r=!(1/a);}//////!!!!()*****+,,,,,-----1122=[[]]cfffffhhhhhhhnoooooorrttttt{{{{{{}}}}}}

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C, 333 bytes

i,v;f(S){char*s=S,L[128]={0};for(v=0;*s;)++L[*s++];for(i=-1;++i<128;L[i]&&L[i]-v?v=-1:0)!v&&L[i]?v=L[i]:0;return-v<-1;}/////////!!!!!!!!&&&&&(((((())))))******+++,,,,,,,----00000111122222228888888:::::::<<<<<<<===???????LLLSSSSSSS[[[]]]aaaaaaaacccccccceeeeeeeeffffffhhhhhhhhiinnnnnnnnooooooorrrrssssssttttttttuuuuuuuuvv{{{{{{{}}}}}}}

Even the bytecount is non-discriminating!

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