* They have the same last name. This is the most straightforward and general way to say it.
* They are related. This implies that they are family, such as siblings, cousins, or a parent and child.
* They are from the same family. This is similar to "related" but can also include extended family members.
* They share a surname. This is a more formal way to say it, often used in legal or academic contexts.
* They are coincidentally named the same. This is used when the people are not related and it's just a coincidence that they have the same last name.
* They are homonyms. This is a more technical term used when two people have the same last name, but it is spelled differently. For example, "Smith" and "Smyth."
The most appropriate way to describe the situation will depend on the context and the specific information you want to convey.