Some bugs have more than one scenarios to reproduced and its not always possible to find out all the scenarios at a time or by one tester. So only By comparing expected result and actual result is not enough.
We also need to check steps to reproduced.
In mercury quality center we can search the defects logged by other testers.
In Set/sort filter dialog, we can give parameters which match with the defect in Summary field,
filter by module
I am getting java script error when logging into site/portal
To track the defects already logged on this:
Open filter dialog:
select summary field and enter praramter like : *scripte* and *error*
and click on ok. Then it will display results, by comparing the defect summary, steps to reproduce we can identify the duplicate defect.