Before advancing our discussion on the subject at hand, let us return to some concepts regarding indefinite pronouns, more precisely the meaning that some of them express. Let us therefore analyze the differences expressed by “someone and someone” and “nobody and no one”. While the latter denote a negative sense, the former express a positive sense. Thus understood, let us proceed by analyzing the following statement:
I didn't see anyone.
When analyzed, it must be stated that the issuer ended up seeing someone, that is, no one. In fact, this is a mistaken statement, given that the functioning of the language must be conceived in a different way, in this specific case.
According to the case under analysis, the negative appears before the verb, which is why we infer that we do not have to affirm before denying, unless that it is a variant that normally occurs in the language – the so-called diatopic variations, whose changes operate at the level of the syntax.
Such occurrences are manifested like this: “I don't know”, “I didn't see”. Thus, it appears that the negation appears after the verb. It should be stated that these constructions are correct, but they result from the unconventional aspect of the language, demarcated, above all, by the orality, in which we can attest to the emphatic use in which the second negative prevails over the first, as in: "I don't know, no" / "no did not see".
Thus, as stated above, in case the negative precedes the verb (I didn't see anyone), it is considered to be perfectly suited to the standard molds of the Portuguese language - which configures the duo negative.
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/dupla-negativa.htm