%%USERID%% Is getting multiple users

[GC-ABlyth]

I am having an issue with a report designed to be run by end users.

The reports get data based on the current user %%USERID%%. Most of the time the reports work fine, however if two users run the same report at the same time, it displays both users data to one of the users.

I have tested this by creating a basic SQL query then have two users access it at the same time.

This is the query used for testing:

SELECT u.firstname
FROM {user} u
WHERE u.id = %%USERID%%

This results in one of the users getting two names, and the other just getting their name.

Is there any thing I can do to prevent this, anything I am doing wrong, or is this a bug?