Middle-clicking an app icon on the Dash always opens a new window, even if app doesn't support multiple windows
When launching apps from the Dash, apps that do not support multiple windows (as per their .desktop) file do not get a "Open in New Window" item in their right-click context menus when the app is running, and Ctrl+clicking the app icon of a running app won't open a new window. However, this restriction does not apply to middle-clicking an app icon, which will always open a new window.
For example, when Gnome Calculator is opened, Ctrl+clicking its app icon in the dash does not open a new window, but middle-clicking it will.
A middle-click on a Dash app icon should behave the same as a Ctrl+click, for consistency.