I put "à" because I am referring to myself.
What should I do, please?
Translation of the sentence: I do not resemble anybody except junior.
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I do not resemble anybody except junior.
Je ne ressemble à personne sauf à junior
Is this what you meant? What you wrote in the title is more like
Nobody looks like me except junior
but worth mentioning that as stated,
Personne ne me ressemble pas sauf junior
it is incorrect owing to the redundant « pas ».
Personne ne me ressemble pas sauf junior.
is very odd and, due to the double negative, literally means:
Nobody doesn't look like me but junior.
which can be simplified to:
Everyone look like me except junior.
Tout le monde me ressemble sauf junior.
That's the opposite of your expected meaning:
Personne ne me ressemble sauf junior.
Note also that naming someone (presumably your son) junior is also uncommon in France.