Quote
users that get more than one answer are recorded in a separate table, with one record per answer. Users.ID and Conflicted_Users.UserID are equivalent.
I want to count the number of users that took the test regardless of whether they're conflicted
If that's so, you can ignore the conflicted_users table.
But you show a row in conflicted_users that does not appear in users, so the data you posted contradicts the description you provide.
Same self-contradiction problem with the data model---it makes
zero sense to record the first test score in one table and subsequent test scores in another. You need something like ...
users(userid PK)
test(testid PK, userID, score, date)
So the answer is, fix the data design.
Till then, if the posted data is correct use ...
select count(distinct userid) from (
select distinct userID from users
union
select distinct userID from conflicted_users
) u;
and if the description is correct, use ...
select count(distinct userid) from users;