Can a blood type O be born from AB and A parents?
The ABO blood type is controlled by a single gene (the ABO gene) with three types of alleles inferred from classical genetics: i, IA, and IB. The IA allele gives type A, IB gives type B, and i gives type O. As both IA and IB are dominant over i, only ii people have type O blood. [1]
So from this standpoint, normal Mendelian genetics cannot explain the blood-group 0 offspring, given the AB parent.
However, besides errors in testing, the (pseudo?) 0-bloodgroup could be explained by the Bombay phenotype. I don't know the incidence rate in your region, so I can't comment on the likelihood.
To give theoretical background, this would be considered a pseudo 0-phenotype, as the individual genetically does not need to be ii. Instead, the individual lacks another effective enzyme that is required for the A or B alleles to be effective; the blood-type specific sugars (N-Acetylgalactosamin|Galactose) cannot be attached to the red blood-cell membrane, as the anchor of these sugars (called H-substance) is not present.
Finally, to address your hypothesis of a gene-duplication followed by a gene knock-out: There is no need for a preceding duplication. The more probable cause is a single point mutation (or multiple) near the active center of the inherited A/B enzyme, or a nucleotide insertion/deletion in its coding region, causing a frame-shift. A completely (not partially!) inactivated A/B enzyme would effectively represent a novel 0-genotype.