Need formula to show staff, appearing at multiple sites

Hi all,

I've tried google and Forum searches, but i'm struggling to find something that I understand and that does what I need.

I have a table in sheet 2 that has data showing which staff work in which sites. (over 54000 rows). Staff Unique ID is in B, and Site is in I.

What I've done is pulled out the unique staff id's (just over 21000), and I'm hoping in my new table, to flag any staff Id's that work at more than one site...

The site names are all unique.
there are expected to be entries where a staff member (rowB) has entries in the same site(rowI) more than once, this can be ignored (i.e. STAFF2 works in SITE 1, SITE 1, SITE 1, this is fine)
I need to flag the ID's (rowB) that appear in SITE 1(rowI) and SITE 2 (and maybe SITE 3 too)

Can anyone guide me towards (or provide if you're feeling really special), the code, formula or function I would need to accomplish this?

I'm sorry that I can't provide sample data... and hope I've explained it well enough here.