The @Function, @Unique, is designed to generate a random, unique number as text data type. When used in a computed field on a form, for example, you should get a new, unique number for each new document created with such a form. You notice, however, when @Unique is used in a Web Application form, the number is not always unique. Why is this happening?

In the case where @Unique is contained in a computed field in a web application, the reason for the non-unique number could be related to the browser cache settings. If the “Check for newer versions of stored pages” option in the IE browser is set to “Automatically”, the number generated may duplicate.

To work around the issue, do one of the following:

Change the “Check for newer version of stored pages” option in IE (found under Tools –> Internet Options –> General tab –> Setting button) to “Every visit to the page” to ensure that a new and unique number is generated by the IE browser. In this case, the Domino server is sending a new and unique number, but the browser was caching the previous value instead of retrieving the newly generated one.
Use a profile document to generate a random number. Then, use a web query open or web query save agent to obtain a unique key by accessing a profile document for sequential unique numbers

Lotus Software KnowledgeBase Document# 1191209