Term reference field GUID - how to make terms unique?

I'm having a problem with term references that are being imported. My example looks something like this:

1st imported node
----------------------
Subject: Science
Type: National program
----------------------

2nd imported node
----------------------
Subject: Science
Type: National program for disabled youngsters
----------------------

"Subject" in this case is a taxonomy term reference. Since the subject on both nodes is "Science", both nodes will reference the same exakt term, but in this case, they shouldn't, because even though the name is the same, there is actually a big difference between them. In the api (from which feeds imports), there is a distinction for this, using unique ID's, but I can't figure out how to use the unique id to tell feeds that those terms aren't the same.

I've tried by setting up a second import that gets triggered by the subject field. The second import creates the taxonomy terms needed, and does so perfectly, making them unique, BUT, I can't get the first feed importer to reference them by doing so (which actually works for nodes, using entity reference).

Does anyone have any idea how to do this?

To summarize (TLDR): How do you import term references (and terms) uniquely, when the term names are identical?