Similar to this thread except we have used Salesforce Contact Id: Mobile Push SDK Functionality
We are implementing MobilePush some time after the initial implementation of Sales Cloud and Marketing Cloud and used the Salesforce Contact Id as the Contact Key. Everything has been working fine inc. journeys and email history flowing back in Sales Cloud using Marketing Cloud Connect.
Now we have the issue presented that our app doesn't have a reference to the Salesforce Contact Id so device registration will create duplicate contacts in Marketing Cloud for known customers in Sales Cloud.
What do others do in this situation? We can still be functional this way but ideally removing the duplicates and updating the MobilePush data views to use the Salesforce Contact Id would be best.