A one liner:

perl -i -pe '!/\w/ or $f {$&} ++ or $_ = ":$&:\n\n$_"' your_file
[download]

Or:

perl -i -aF'(?<=.)' -pe '$F [0] =~ /\w/ or $f {$F [0]} ++ or $_ = ":$F
+[0]:\n\n$_"'  your_file
[download]

Abigail