Strange key column and duplicate behaviour

[Emmanuel_Maguer]

I am trying to solve an issue of duplicate key in the table where data is recorded from forms with multiple users: Thanks to the help of this community, I have created a virtualcolumn with intial value DateTime NOW() & [username] which for me makes my key unique. It should be impossible for a user to open twice the same form at the same time.
But it is the case; Today I had a duplicate key “08/22/201912:07:11John”. How that can be ??

Thanks for your help.