An item with the same key has already been added

Error when try to open an alert properties:

An item with the same key has already been added. (mscorlib)

Cause: There are 2 (or more) jobs with the same name.

Find the duplicated names of jobs using this query:

select name, count(*) from msdb.dbo.sysjobs

group by name having count(*) > 1

Fix: Rename or delete one of the jobs with the duplicate name.

* Alerts can be executed by jobs, so maybe it caused an error when try to find this connection between alerts and jobs.