While that might happen (and probably cause a method redefinition), there is an important convention that helps preventing it: your package must either own the function or at least one of the types used for arguments, otherwise you're practicing type piracy [1]. I've seen an automated scripts that can detect type piracy, so hopefully it could be part of a linting toolset eventually since not everyone might be aware, but at the very least popular packages shouldn't have them - or at least not in a way that may cause bugs (and if any package has it unintentionally it's probably worth creating an issue).

[1] https://docs.julialang.org/en/v1/manual/style-guide/#Avoid-t...