A double hammer is a kind of tool that has two hammers on one handle. Imagine you have two hammers next to each other that are best friends and they want to always stick together. So, they decide to join forces and become a double hammer!
This tool makes it easier for people to do tasks like hammering nails, because instead of having to pick up two hammers separately, they can just use one tool with two hammers. It's kind of like having two hands to do a job, but it's just one tool.
So, if you need to do some hammering, a double hammer can be a really handy tool to have!