Well, simple this is not. Some people can be made of "two" different DNAs. It basically happens that two embryos fuse making one health adult with two different DNAs, also known as a chimera.
If such a man has mixed sperm, meaning that some sperm is "his" and some are from a "melted, merged, mixed-in" brother. What happens to the children, for example, if some are born with DNA 1 and others are born with DNA 2?
Also, what about a cohen in this situation?
In summary, what's the status of such a "man", "child" and "mother/wife", also, consider the case of a cohen born with such mix and his wife, is she "cheating on him" and or "having children from two different people" etc?
This is an article presenting a case that happened recently where paternity tests said the child was his "uncle's" etc