You want the ability to identify defects raised by multiple testers and to do so efficiently.
Assuming all testers are logging their findings in a common defect tracking database the standard process is:
1. Anticipate defects that could likely be reported by multiple testers
2. Conduct a query in your defect management and reporting tool based on most likely keywords.
3. Consider this method isn't foolproof because the language testers use to report defects invariably differ and some similar defects will be missed by your query. Therefore you may need to be creative to discover all of the same defects.