A man (non-dad) conceived a child that did not match his (non-dad's) genotype, but the child's DNA was consistent with the child being the grandson of dad's parents. The proposed explanation is that dad had a twin brother (twin-dad) who was never born but whose cells colonized non-dad's testes when non-dad was a fetus. The cells of twin-dad produced the sperm that conceived the child.
In the modern era of sensitive genetic testing, multiple examples of chimerism are being detected where chimerism refers to a body containing cells derived from more than one fertilized egg. All of us probably contain replicating cells from more than one member of our genetic family. A distinction should be made between bodily individuals (who are chimeric) and genetic individuals who may be distributed across multiple bodies.