Twins
There is no need to worry, you are not seeing double, instead, you are just looking at a very rare case of twins. So these two brothers who are twins married two sisters, who are always twins. Then they both had two boys who are technically cousins, but because they are so close in genetic makeup, they technically are brothers.
Are you confused yet, well don’t scratch your head just yet until you hear the final piece of information, they all live together? That is a bit weird for us, how do they know who is who and which kid is theirs? There must be some massive confusion going on in that household.