Should I update to Sierra 10.12 if I already have it? I am confused by my macOS: it proposes to update itself to the same version it is already.

Should I do that?

There was Sierra beta version installed on the iMac some time ago; maybe that's the cause.

Current system version: macOS 10.12 (16A323) and I don't know if it's beta or not.
Is this just showing at the top of the window in the App Store? I think that will show the latest OS version independent of what is installed on your system. If it is coming up as an update, in the "updates" tab, that's a different story - if so, install it, otherwise ignore it, it's just an advertisement.