A somewhat misused term that simply refers to the formation of a red blood cell antigen from the action of two identical alleles (i.e., the person is genetically homozygous for that allele). This is further outlined in the discussion on the Dosage Effect, including an illustration that should make things more clear if you don’t understand the above. Since most blood group system genes show codominant inheritance, a person who gets two copies of the same allele will usually have more antigen than someone who only has one copy of that allele (we refer to the antigen in the latter case as having a “single dose“). Blood bankers must recognize whether or not an antigen is “double dosed” as part of the evaluation of an antibody panel.