include question

[Dill, John]

Hello all,

I have a question about the include directive.  I have two files, where one 
includes the other.  In each of these files, they both include a common file.  
Is that common file included twice?

example

In file 'header.mk'

include defs.mk
...

In file 'source.mk'

include defs.mk
include header.mk
...

Is defs.mk included twice?  I have a few makefile functions that I've 
implemented which build off of each other, and I have tended to include every 
file for which a user-defined function is used.  But I find this is affecting 
the make response time, in the realm of seconds now on a slower machine.  My 
guess is that this can be due to multiple inclusion of the same files.  Is this 
a possibility?

Thanks,
John