[FEATURE REQUEST] add option to ignore duplicate includes

[tomdoherty]

Is your feature request related to a problem? Please describe.
This feature allows moving include statements into macros which are used often.

Describe the solution you'd like
An option to ignore the ConstructorError when a duplicate include is defined.

Describe alternatives you've considered
At present we work round this via a custom renderer.

Additional context
N/A

Please Note
If this feature request would be considered a substantial change or addition, this should go through a SEP process here https://github.com/saltstack/salt-enhancement-proposals, instead of a feature request.