But do you actually have it selected? I am not an expert myself but only because you have one disk does not mean that it is automatically selected.
Yes, it is selected, but I selected it again just to make sure. Still got the double boot thing. I don't know what it's doing while the screen is light grey (without the Apple logo) but it seems to take forever - it's somewhere between 30 and 45secs. And, of course, it does it twice so it's quite a nuisance.
So I'm now interested in 2 things:
1. Why the double boot?
2. What's it doing while on the light grey screen?
Other than that, everything's fine. Just a reminder - I'm still on HS, but I think these issues would apply to Mojave too.
Update:
I thought I'd start up in verbose mode (Cmd-V) - knowing that the messages fly by so fast that it's almost impossible to spot anything meaningful. This is what happened:
chime, light grey screen (no logo) for about 45secs, apfs patch output, verbose mode output, grey screen with logo and progress bar, normal after that.
So, a couple of things to note. It still did the first grey screen, so Cmd-V had no bearing on that, and I still don't know what it's doing during that time. There was no second chime before it it did the verbose mode output.