avoiding duplications when using _path_files in completion function

[Paul Seyfert <pseyfert@mathphys.fsk.uni-heidelberg.de>]

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Hi,

I'm writing a zsh tab completion function for a program which should get
started as:

PROGRAM INPUTFILE1 INPUTFILE2 [possibly more input files]

Typically one doesn't want to provide a file twice, so in a directory
with two files

cumbersome_long_5_filename.extension
cumbersome_long_X_filename.extension

i want to only suggest files which don't occur in the current line
already:

PROGRAM cumbersome_lone_5_filename.extension <TAB>

- -> suggest only cumbersome_lone_X_filename.extension

I imagine this is desired in other places as well and solutions exist.
So far I'm using (after some if for other use cases and non-filename
suggestions)

_path_files -g '*(/) *.extension'

which does suggest files several times.
I tried the -F option (for testing when suggesting the second filename,
providing the first filename to -F):

_path_files -F "$words[2]" -g '*(/) *.extension'

but this keeps suggesting the first file a second time (in zsh 5.2).
Any suggestions?

Thanks in advance,

Paul
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