it means you know it before I did.

complete word is：你（一）早就知道了吧？ 一早 是 一大早 的延伸意义，but no the same. 一大早 又是 一个大好的(great good = excellent)早上 的 Simplified form = early in the moring try to imagine a scene: you get up early, and you stretch in a big and empty square. The sun was only just coming up.Dew in grass has not still dried, most people still in the bed. and you will feel why use 大 to describe this quiet and beautiful moring. that is a habit of most chinese people above. so，你一早知道了吧？ imagine： you know sth in the morning? 延伸意义：= you know sth early/previously, because you know this in the moring （just Abstract meaning, no really）.

It means you have known something already.

It means "you" know early or you already Known

Literally, "早" means an earlier time here. So, ”你早知道了吧？“ means "You already knew it, right?" 希望我的回答对你有帮助！ :)