This depends on the application and is also affected by other ways of launching apps (i.e. command line or keyboard shortcuts), so I'm not 100% convinced it's only a Slingshot issue.
For example, apps like Music, Videos, and Photos don't spawn new windows but focus the existing instances. Midori and Terminal open new windows, while Files opens a new tab in the existing window.
This depends on the application and is also affected by other ways of launching apps (i.e. command line or keyboard shortcuts), so I'm not 100% convinced it's only a Slingshot issue.
For example, apps like Music, Videos, and Photos don't spawn new windows but focus the existing instances. Midori and Terminal open new windows, while Files opens a new tab in the existing window.