Q:

How to ensure single timer trigger azure function in host

EDIT
I am surprised I missed this detail in the documentation. Regardless of instances it will only run a single timer.
I'll probably delete this question given that it is my oversight in the documentation and I'm polluting SO. Thanks for your answer, I hope you can keep your points.
https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-timer#scale-out
Original question
Is there a way to ensure only a single timer function is ever run (or active) when the script host has created many instances of your function app? 
I have other functions that cause the function app to be scaled out with many instances and I am assuming each of those instances will be triggering my timer function at other intervals offset by their instantiation time. 
If not, what is the ideal Azure service to use as an external timer to call a refactored Http trigger function to complete the same unit of work?

A:

This safeguard is built into Functions. By design only one instance of a timer-triggered function is active regardless of the number of instances your function app has scaled out to.

