Hello Guix,

The attached patch (for core-updates) removes duplicate entries from 
search paths.  I believe this is safe.

I was encountering "command line too long" errors while trying to build 
several higher-level perl modules, and noticed that the values being set 
for PERL5LIB, PATH, and others were ~318kb each.  Investigating further, 
many of the entries were duplicates, resulting from the many propagated 
inputs that are needed in perl modules.

While it's possible that, even with this patch, we could encounter the 
same errors in the future, it hasn't happened yet, and I've been able to 
continue packaging modules for hydra.  If it does become a problem 
again, we could consider patching the perl modules themselves to point 
to their dependent modules, thus alleviating the need to propagate so 
many inputs.

`~Eric