Huh? There is literally no evidence or even allegation of that. The person was paid to collect petition signatures, so she fraudulently obtained petition signatures. Which obviously are way less closely tracked than actual votes.

> So what? If it was illegal to register multiple voters at the same address then it could have been detected at the registration time.

Well it's not illegal to register multiple voters at the same address, obviously. It's illegal to vote under someone else's name. A bunch of votes coming in from a single residence would be flagged. Should voter registrations from a single address get flagged? Sure! And they probably do! But as you say, that's not a crime. Voting fraudulently is, which is not even alleged here.