Identify duplicate records in a subform

[Cornfused]

frm_courses, sfrm_participants

When I enter a new course into the frm_Courses and begin to enter names into
the sfrm_participants, I'd like for those attendees that have attended a
course with the same name/code to be identified in some way.

Other info:

sfrm_participants is displayed in datasheet vew only
I have a duplicate attendance query (qry_Duplicate_Attendance) that returns
Attendee name and dates of this attendee having attended, but I haven't
figured out how to put these things together in a manner that identifies the
attendee on the sfrm_participants.

Coniditional formatting is a possibility, I guess, but I haven't been able
to figure out how to construct the function I would need.

Please help.

 

[Cornfused]

Forgot to mention that the the participants' employee ids are also on the
sfrm_participants. This datapoint should be easier to deal with than names.