Background check after receiving FT offer

Hi guys,

I recently accepted a FT offer from a top BB (GS/MS/JPM) after my summer internship. HR told me there will be another background check before joining as full-time.

Thought it was weird as they are asking for the exact same documents / info. as they did during the background check before I joined as a summer analyst.

So my questions are:
1) Why do they go through the exact same (and tedious) process if they have already done so?
2) Don’t they keep my files? Was wondering cuz the HR seems to be asking for the exact same documents
3) Is this normal?

Thanks!