Hi there
I've just been doing some testing of this on my own local site, and I haven't experienced any problems with duplicate font requests.
I have a page that I added a Toolset Heading block to.
Initially I didn't specify a font in the Typography settings, which means when displayed on the front end the heading simply uses the styles defined by the theme, including font-family.
If the theme specifies font-family rules that include a list of fonts with fallbacks, and includes locally installed system fonts (like arial or verdana) then they would be used.
I then updated the Heading block to use some Google font.
When I loaded the page on the front end there was a single request to the Google API for the font. Subsequent page loads (of the same page, or other pages where the same font is used) would use the font from the browser cache memory, and wouldn't hit the Google servers again (unless, during testing with the browser console open you have disabled the browser cache).
If the same font were requested twice on the same page (which I'm not seeing, but it sounds like you may be) then the second request would simply be served from the browser memory cache, the font wouldn't be retrieved and downloaded a second time (unless the browser cache is disabled).
I also tested with GeneratePress and didn't experience any problems.
One thing to watch out for, though, could be if GeneratePress loads Bootstrap and you also have Toolset set to load Bootstrap.
In any case, you can not have Toolset load any Google fonts by not setting font-family in the Toolset block settings, and instead let the Toolset content font be determined by the theme. (If the theme styles don't target some Toolset content you can used the Advanced tab to add an ID or Class that you can use to apply style settings in CSS you add to your child theme).