Duplicate version of Mail launches invisibly

In the last week I noticed the following. I'll launch mail via Spotlight or the Dock. Mail opens. Then while I'm doing ordinary activities, I'll close a draft version of an email. Directly behind it is a second version of that draft email. Or I'll minimize the Message Viewer window (the main window containing the column of mailboxes on the left and message preview on the right). When I return to the app later, a second version of the Message Viewer window is open and active. This tends to happen while I leave Mail on for a while - put the system to sleep, wake it up and so forth.

When I check the Dock, there are two versions of Mail. One is where I keep it stored, and the other is at the end where apps not fixed in the Dock appear. If I select either of them and say Options>Show In Finder, the same finder window opens to the Applications folder showing the Mail app. Same one. When I quit one, the other one stays live.

Any guesses as to why or how this is happening?