If you change the prototype trigger key (expression), on next discovery Zabbix will leave an old trigger with the same key (expression) and create a new one besides it.

To reproduce this bug:

1. create a prototype trigger which uses one key as a trigger expression
2. wait for the discovery to execute to create this trigger on a host
3. once the trigger is created, edit the prototype trigger and choose a different key in the trigger expression, but leave everything else the same
4. wait for the discovery to execute again and you will see that the host now has two triggers with the same name, but with different trigger expressions.

I haven't tried if this happens only of I change the trigger function and not the key.