Xcode no longer kills the old process when starting a new debug (Mac)

Usually when I am running/debugging an app, pressing the 'play' button to go again will kill the old process and start a new one. But recently XCode has been leaving phantom processing running when I do this. I have three or four of the same app processes in the dock. The old ones are not really running -- they don't do anything when I click on them. But I have to Force Quit them to get them to go away. Anyone know what causes this, how to prevent it?