Why is the penicillin/ceftriaxone hypersensitivity test only done once?
Because developing a hypersensitivity takes time. If you get exposed to an allergen, the body first has to produce those antibodies that then get used in the hypersensitivity response at the second, or subsequent, exposure. 

If you are talking about taking a second test just minutes after the first one, how would your body even "know" that this is a second exposure? Two exposures within minutes are not distinguishable from one exposure for the body. 

Usually, an allergic reaction shows the second time a patient takes a full course of antibiotics. It does not show at the second oral dose or injection, which are way too close to the first dose to have already lead to the developing of a hypersensitivity. 

So, why test you in the first place? Because you might have been exposed previously without remembering it - people misremember what medication they got during their childhood all the time, I imagine, and your doctor/nurse seem to just routinely do the test before giving these antibiotics. 

Sources/Reading

Drug allergies

Update on the Management of Antibiotic Allergy