Hi @rory99,
Thanks for updating. It is also working fine on my side.
For your another flow, could you share a full screenshot of it?
Do you get Due date from the action Get items? If there are multiple items that match the condition, multiple notifications will be sent out for each item.
I have made the following test, only one email was sent out when there was only one item matches the condition.
In action Compose2, use formatDateTime function to format the due date to “yyyy-MM-dd”:
formatdatetime(addDays(outputs('Compose'),7),'yyyy-MM-dd')
In Compose3, get Today’s date with format “yyyy-MM-dd”:
utcNow('yyyy-MM-dd')
Then send out the email if output of Compose 2 equals output of Compose 3.
Image for your reference.

Best regards,
Mabel