How to make sku unique?

Question

I just found out I have duplicated sku. Not sure how this could happen since AvS_FastSimpleImport with Mage_ImportExport_Model_Import::BEHAVIOR_REPLACE should always replace those products. Looks like I did messup one import last year. All product have the same date for getCreatedAt().

Anyway, if I look at sku attribute config I can not change Unique Value to Yes. Is this not possible in general or only because I have duplicates?