The htaccess in my fresh zenphoto installation has a double slash at the RewriteBase.
What's it all about?

I found this, and the link is dead:
https://github.com/zenphoto/zenphoto/issues/1025

Because the issue is already fixed on GitHub. And there will soon be an update.

I don't understand.
The Issue is from Jan 29, 2017.
And the Zenphoto 1.4.14 (Jan 28, 2017).
In what release is it are fixed?

Is it a bug? Can i remove one slash?

Master on Github => coming release. We are a bit behind with the next bug fix release. And yes it is/was a bug. You can of course remove it manually from the htaccess file, too.

OK.
Yes, i have found another bugs there was always fixed in the github version.

That's why we recommend to try the support build first (not on a live production site naturally!).