[jQuery] jQuery in e107 plugin?

[Harsányi János]

I have two e107 plug ins which are using jQuery.
The first one uses jQuery UI too.
The problem is that the second plug in re-defines the jQuery without
the UI extension, and the first plug in is not able to work properly.
Is there any way to load jQuery only if it has not been added yet?
(I have tried typeof(jQuery) but it is throwing an error: "jQuery is
not defined".)