When Filemaker fails to prevent duplicate records being created

There is a good description of how to set up validation in Filemaker in order to prevent dulicate records being created here: http://fmforums.com/forum/showtopic.php?tid/176348/post/204677/#204677. This post is concerned with the cases in which even these methods fail.

We have implemented option 1 in the above post, defining a unique key for each record and using the Unique validation requirement to ensure that no duplicates are created. However, we still occasionally get duplicates being created. Does anyone have a list of scenarios in which this can happen?

In our case, the records in question are all in "incidental" join tables and are created from a parent record as a related record in a portal, typically by selecting an option from a drop down list. In normal use, attempts to create a duplicate prompt the standard "Revert" warnings. Nonetheless, some duplicate records appear from time to time.

The only explanation we can think of is that occasionally the user may force quit the application while the Revert warnings are on screen.

Can anyone confirm this, or cite other scenarios?

Then the question will be, what can we do about it?

many thanks in advance,

W.