Does #pragma once insure that the header file it is included in is only called once AS WELL AS that the headers which are included in said header file are not yet included?
The pragma doesn't affect other headers. if the header with the pragma 'a.h' includes 'b.h', 'b.h' can be included again via a third header or directly.
Also, if it is only called once, does that mean a .cpp file that needs a particular header will not be able to access it?
You can include the header from anywhere you want, as many times as you see fit.
If a header file is marked with #pragma once and included in a .cpp, can that header file be used again elsewhere?
Yes, this is the normal practice with headers.
Where's the catch?
If you really need to include the headers more than once and every include performs a different operation than don't use pragma once or a sentry macro. These cases are not common.
A benefit to pragma once
is that it saves you from bugs like having 2 header files that by chance have the same sentry macro. this can happen when 2 header files have the same file name and same coding style for macro names.