Identical twins share nearly 100% of their DNA, which can make them extremely difficult to tell apart, even with genetic testing. In rare cases, if identical twin brothers both have children with the same woman around the same time, standard DNA tests may not be able to determine which twin is the biological father. Similarly, when identical twins marry another set of identical twins, their children are technically cousins, but genetically they are as similar as full siblings.

Source: one, two