A vim command that will take a list in a buffer and find & remove 1 (or more) duplicate lines (instead of using `:sort u`

:g/^\(.*\)\n\1$/d

:g: The global (g) command in Vim, performs a certain action on lines that match a given pattern.
/^: This symbol denotes the start of a line.
$ and $: These parentheses are used for grouping in regex. Everything between $ and $ is treated as a single group. This group can be referenced later.
.*: The dot (.) stands for any character except a newline. The asterisk (*) is a quantifier, meaning "zero or more of the preceding element". So, .* matches any string, including an empty string.
\n: This represents a newline character.
\1: This is a backreference to the first (and only) group that we defined earlier. In this case, it refers to the exact string that was matched inside the $ and $.
$: This symbol denotes the end of a line.
/d: The d command deletes the lines that match the given pattern.

In summary, the regular expression :g/^$.*$\n\1$/d performs the following operation:

It looks for lines in a text that are immediately followed by an identical line.
If such pairs of identical lines are found, both lines are deleted.

OR

:%sort u