remove duplicates from trailpack prerequisites

[tjwebb]

Issue Description

If a trailpack config listens for a duplicate event (or, manually adds a system event like trailpack:all:configured) into the config, after() will listen for two occurrences of the event and the app will not load.

Environment

• node version: all
• trails version: all
• operating system: all

[scott-wyatt]

I could probably start on this task if you point me in the right direction.