the knot will tie itself again in a few seconds

The knot should look like this when you are done

Two half hitches can be used to secure an animal post, a boat to a dock, or to suspend a hammock between two trees. However, the more you pull on this knot, the tighter it becomes, and the harder it can be to untie. Thus, two half hitches should never be used to rescue someone. Use a bowline instead.