Q: If I setup Alert Central to go hand in hand with Orion NPM, will AC take over the email alerts?
A: No, it doesn't. So if you set up some email for Alert on NPM side (say, email@example.com) and then set up a rule on AC side so that alerts from NMP are sent to some email (say, firstname.lastname@example.org), then when a node goes down, both emails will be notified.
Q: Will all my emails be coming from AC only?
A: No, will be sent to both emails. See details above
Q: If I setup a custom email in NPM, will that show in AC as well?
A: No, only alert properties will be shown in AC.
So we if you'd like not to be notified twice, you'll have to switch off email notification on NPM side