How do I initialize globalized minor mode only once?

[Dmitry Gutov <dgutov@yandex.ru>]

Hi all,

I have written a globalized minor mode which has a relatively costly
initialization (external process call), which I need to do when the mode
is enabled.

But the turn-on function is getting called twice for each new opened
buffer, once in default major mode, and once in the final major mode.

What's the best way to turn on the minor mode only once?
Currently I'm comparing major-mode value to the default value, but
that's probably not the best solution, since the minor mode in question
can apply to some fundamental-mode (or other default mode) buffers, too.

Regards,
--Dmitry