Tiny system font size disaster

Help, my system font has become tiny and is far too small for my work. I can now only work properly with a large external monitor attached. (I'm running El Capitan on 15” non-retina mid 2012 MacBook Pro).

This problem started yesterday. I had a very minor problem in that Pages does not always start the first time. Somebody recommended checking whether there were damaged fonts in Font Book. I did that and found many cases where common fonts were shown as being duplicated, which the system allowed me to correct automatically.

I don’t know what was actually done automatically and cannot find anything in the trash, but since doing this the system font has become tiny.

I have compared the following 3 locations on the hard drive with the same locations on my external boot drive. (I clone my entire drive weekly using SuperDuper to an external drive for emergencies). The number of font files is identical in each case.

~/Library/Fonts/

/Library/Fonts/

/System/Library/Fonts/

How can I get my system font size back again please or reverse the corrections which the OS made to Font Book?

Might it be sufficient to replace the newly written fontbook.plist?

Does anybody know a solution please? This is a disaster for me and a huge set back if it cannot be solved. Thanks!