Implement a deduplication trigger in Oracle
This is not the sort of task that you would want to use triggers on tables for.

If you really wanted to use a trigger, you could declare t_alarms as a view, rather than as a table.  You could then write an INSTEAD OF trigger on the view that translated INSERT statements on the view into UPDATE statements against a new base table.

Were it me, though, I'd let the INSERT statements go into a table which allows duplicates and then create a materialized view on top of that table that does the aggregation.  You can have that materialized view refresh ON COMMIT if you want to ensure that the data in the materialized view is always in sync with the data in the detail table.