I have multiple use cases for that but let me describe a simple scenario:

1. Log file output
Log file output: ERROR: File not found /path/to/file

2. Trigger is setup to get that error and extract the path to the file to a tag called FILE.

3. Now if the file /mypath/myfile already has triggered an open problem i would love to prevent multiple notifications about the same file. This could be done by verifying if a problem is already open that has the same value(s) for the existing tag(s)