Lookup field within same form?

Lookup field within same form?

I have a complicated problem I'm trying to solve with lookup forms.  Let's say I have three forms: Concerts, Pieces, and Composers.  Pieces is related to Concerts (there are pieces in concerts) and Composers is related to Pieces (composers write pieces).  I want to create a lookup field within Pieces so that when I use a subform Pieces within the form Concerts I'm not accidentally re-entering pieces that already exist (I've checked off "no duplicate values" but since I'm entering pieces from all over the world, they might be spelled slightly different at times and there's no other way to be sure other than a lookup field).

I'm getting the impression that a lookup from within the same form isn't possible: can anyone think of a workaround for this?