Which is exactly what Opus is doing. As several people have tried to explain. The only difference being that each lister is opened by the same initial Opus process...
Opus doesn't randomly open listers to annoy you - if you only want one open then don't open any more. There's no need to have a setting to prevent this...
I really don't see what's so confusing here Opus runs and sits in the tool tray, each window or lister is opened as required by the user. Some prefer one lister, some prefer one dual pane lister, some prefer multiple listers, others use a combination of the lot. Simple.
It occurs to me that people with small monitors might see less benefit from multiple listers than those with decent sizes. For an extreme example, see Leo's screenshots.
I don't see what else can be said to explain this to you - it's nearly time to lock this as it's taking up a lot of time for little gain.