[BUG] _describe -x adds same completion group header twice

[Marlon Richert <marlon.richert@xxxxxxxxx>]

When _describe is called with -x, it adds the same group header twice:

% autoload -Uz compinit; compinit
% zstyle '*' group-name ''
% zstyle '*' format '--%d--'
% compdef _tst tst
% _tst() { local -a arr=( foo bar ); _describe -x 'desc' arr }
% tst \t
--desc--
--desc--
bar  foo