MS Access drop down boxes misbehaving on copy

I’ve two access databases, each with identical tables and fields, the only difference being that one refers to the timetable for 2009-10 and the other for 2010-11. I want to join the data from the tables called Weeks in each DB, but when I copy data from one Weeks table in the 2009-10 DB to the Weeks table in 2010-11, the drop down boxes that refer to teachers (referenced by ID number to a Teachers table) misbehave and where there are multiple teachers with the same surname, the copied drop down shows the first teacher with that surname. All the other teachers are fine, it is just where there are multiple duplicate surnames. This happens whichever DB I copy or paste from.

Importing the table as a temp table then copying the data from that (so at least it is copied from within the same DB) has the same results.

Can anyone shed some light on why its doing this or what I could be doing wrong?