How can I make sure that if the user_id is repeated, then not add it to the count?

Question

SQL

such a database

id      |    user_id     |       level_id
1       |        5       |           7
2       |        5       |           6
3       |        4       |           9

Answer 1

You can also do this:
select count(distinct user_id) as cnt
from table

Answer 2

Until you say what you want to count, and on which DBMS, the number of records by unique user_id will look something like this:

select count(*) as cnt
from (select user_id
            from table
     group by user_id)

If you need to display some more nuances in the selection, then the query will change dramatically.