Can you enforce uniqueness in MySQL based on column in foreign table?

I am refactoring one of my database tables and I've come up with an issue. Specifically, I have a table with four columns. We'll call the first three my.a, my.b and my.c. I have a uniqueness constraint on the combination of these three columns. Column my.d is a foreign key to an auto-incremented ID column in another table foreign. I realized that this other table *also* has a column foreign.a and both column as are complete duplicates, i.e., if you join the tables together on d, my.a will always equal foreign.a. Apparently this is part of the requirements. I would like to remove column my.a from my table, but that will remove the uniqueness constraint. Is there a way to somehow set up a uniqueness constraint on the combination of foreign.a, my.b, my.c? Or am I better off just leaving the duplicate column in place?