I had the same issue as the original poster: double click on Windows 10 would not open a Layout 2018 file if Layout was already open.
My fix was to run Layout.exe as administrator.
I’m unsure why this worked. The original installation was undertaken as administrator.
I only have 2018 installed, however the PC did have 2017 at one point in time. Layout 2017 still appears as an option if I right click open as, suggesting is something amiss with remnants of 2017 or file association.