Prevent invalid lines from causing duplicates
bufnr=0 is the alternate-file and not a proper buffer number, so ignore
it. When you have lines in the quickfix without associated files, they
will have bufnr=0. Example:
|| grep: data: Is a directory
Remove unnecessary concatenation operator.
With a hat tip to Tim Pope.
Create a :Qargs command.
Code adapted from DrAl's solution on StackOverflow: