How can I highlight duplicates of unique values including first occurrence if only one other cell in corresponding row contains text?

I'm a school admin trying to set up a school event. We don't have a data specialist, so it's been up to me and YouTube thus far!

Unique student IDs occur 8 times, each time with a different teacher. I need to assign 16 students to each teacher, but must be one of the 8 they currently have for class.Column A has unique student ID (occurring 8x) and Column C has unique name of each teacher.

If I filter H for one teacher, and assign 16 students by copying and pasting teacher name into Column D, how can I highlight all duplicates including first occurrence? This way I can filter out highlighted rows and avoid "double booking."

Conditional formatting with VLOOKUP and if statement? Thanks for any help you can provide; you'll be making my teachers' lives much easier!