Extremely silly, but amusingly short :)

#!/usr/bin/perl -wl
use strict;

my @a=0..1e3;       # Set higher than any reasonable message number
my @list = ("1, 3, 5-7", "1-5, 2, 4", "1-3, 2-5");

for (@list){
    my %h;
    s/-/../g;
    $h{$_}++ for @a[(eval $_)];
    print join " ", sort {$a<=>$b} keys %h;
}
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Output:

1 3 5 6 7
1 2 3 4 5
1 2 3 4 5
[download]

This relies on Perl interpreting your format (once dashes are replaced by double dots) as array subscripts.