I propose we add a toggle option which prevents more than one window from being opened. While enabled, when a user attempts to open a new window while the singleton window already is opened, the singleton window will be displayed instead of a new window.
Rationale: Many times while using speedcrunch I accidentally open many instances of it because I forgot I had previously opened it. On particularly busy/forgetful days this leads to 10+ SpeedCrunch windows cluttering my Alt-Tab. This is especially troublesome when I need a previous result and cannot remember which window I typed it in! Forcing the first window to be the only window would solve all this.
Minor issue: When using virtual desktops I sometimes may want multiple windows (one per desktop). It would be nice to enforce a one-per-desktop rule, but to keep things simple we probably could just stick with one vs. many windows as being a single toggle.