I am importing data from a student information system into MySQL. I
need to create unique usernames in the format of "first.last". If
there is a duplicate, I would like to be able to insert the username
as "first.last[n]". Currently I have 262 duplicate combinations of
first and last names.

I imagine this can be done with some sort of looping structure, but I
would like to ensure that it remains fast (currently importing 10,000+
users in under 10 seconds).

I am thinking of doing the following (not any particular language, but
you should get the idea):
<code>
i = 1
dups = false
dupnum = 0

dupnum = SELECT COUNT(*) FROM students s WHERE s.uername IS NULL

if dupnum > 0 then
dups = true
else
dups = false
end

Do while dups = true
UPDATE IGNORE students s SET s.user_name = CONCAT(s.firstname, '.',
s.lastname, i)
dupnum = SELECT COUNT(*) FROM students s WHERE s.username IS NULL
if dupnum >= 1 then
dups = true
else
dups = false
Loop
</code>

Any thoughts or better ideas?

Thanks!