Only allow 1 discount using commerce_discount I am having trouble finding out how to only allow 1 discount per order.

eg. a specific user role allows 15% discount on their order.
We also have another discount set up that applies a further 15% on a certain product.

We don't want to allow both discounts to be applied when that specific user role orders that product.

We are using: 


Drupal 7 
Commerce 7.x-1.13
Commerce_discount v 7.x-1.0-alpha5
Your version of Commerce Discount is too outdated to support discount compatibility. However, it's so old, that the upgrade process may be complicated. Drupal Commerce itself is also currently at version 1.15, so that will need to be updated, too.

I'd recommend creating a copy of your website and bringing those modules up to date, maybe even one version at a time, running update.php in between each upgrade. (Theoretically, it should work fine to run update.php all at once, but Commerce Discount has to do a lot of field manipulation in its update functions, and I just worry about the potential for things to break...)

Once you do that, you'll have discount compatibility settings that will allow you to enforce a discount being incompatible with any other discount, and by adjusting the sort order on the discounts you can identify which discounts should apply first to ensure the restricted one applies first when possible.