Since the time window, between the execution of the two line, responsible for the race condition, is pretty small. You have to be pretty fast and pretty lucky to actually reproduce this issue (we have only 14 occurrences in 190111 forum subscriptions). Thus, I suggest the following procedure to first reproduce this issue and afterwards test the patch. First reproduce the issue without the patch: Create a new forum and subscribe three users to it. (Lets say your user ids are 2, 3 and 4 and your forum id is 5 ) Go to the database and execute this select statement (this statement is executed more often later on): select * from mdl_forum_subscriptions where forum= 5 ; Result: 3 entry are shown for your three subscriptions. Go to the database and execute the following statement twice: insert into forum_subscriptions (userid, forum) values ( 2 , 5 ); And the following once: insert into mdl_forum_subscriptions (userid, forum) values ( 3 , 5 ); # Now the above listed select query should return 6 entries, including one duplicate and one triplet. Load the list of forum subscriptions, where the mentioned exception is shown. Now install the patch ( Without resetting the DB state ): The above listed select query should return again 3 entries for both subscriptions, but no longer any duplicate. Load the list of forum subscriptions. No exception should be thrown! Go to the database and execute: insert into mdl_forum_subscriptions (userid, forum) values ( 2 , 5 ); You should get an error like: ERROR:  duplicate key value violates unique constraint "mdl_forusubs_usefor_uix" DETAIL:  Key (userid, forum)=( 2 , 5 ) already exists.