I am working on a simple square. At each end, I need to add a what is know as a "dog bone" in CNC jargon. (See image for reference.) (Cutting bits are round, so you need these small adjustments to avoid issues once you assemble a piece.)

I painstakingly added the first dog bone in the bottom right corner, zooming in to the max in order to split the lines by hand. Is there an easier way to do this? And then, how do I replicate this dog bone effect in all four corners?