diagnose duplicate sources (was: found automake bug)

[Ralf Wildenhues]

Hello Andreas,

* Andreas Otto wrote on Sun, Apr 19, 2009 at 11:02:00AM CEST:
> 
>   I wrote the following line into my Makefile.am:
> 
> pymsgque_la_SOURCES = pymsgque.c pymsgque.c pymisc.c
> 
>   -> double include the file "pymsgque.c"
> 
> and got the following compile error
[...]
> -> please check for double argument in Makefile.am and write an error ...

Thanks for the suggestion.  I don't think this is a bug in Automake,
rather a bug in your Makefile.am, but you do have a point in that
Automake could be more helpful here; so this is a quality of
implementation question.

There are some questions to consider: for which set of files is it
suitable, desirable, and safe to diagnose duplicates?

- compiled source files in the same _SOURCES variable: probably safe.
- object files: less clear to me.  There could be compiler or system
  support objects that actually want to be included more than once.
- mere file listings like EXTRA_DIST: undesirable in general (some of
  the entries can be generated by other tools, or added by automake
  automatically),
- with multi-file install, however, it is very desirable to diagnose
  duplicates in one variable, as they may evoke failure from `install',

Sounds like a good post-1.11 project to me.

Cheers,
Ralf