、。
She's been here before, and so have I.
Literal
She [topic-は] before [at-に] here [to-に] came [thing-こと] there-is, I also again so is.
Two standalone observations linked by 連用形 connective あり (the literary stem-form of ある acting as 'and') — 'she has been here before, and...'. The follow-up 私もまたそうだ uses も ('also'), また ('as well'), and そうだ ('that's how it is') stacked for emphasis: 'me too / so do I.' The whole sentence reads slightly literary; everyday Japanese would lean on ~けど or just split into two sentences.