Two common errors that can catch even the more seasoned DNN developers by surprise, which took me the better part of a day when I have to push a site up to production.  Hopefully this helps someone else.

1.  [FATAL] DotNetNuke.Web.Common.Internal.DotNetNukeHttpApplication - System.ArgumentException: An item with the same key has already been added.
   The cause is Module Controller names are mismatched.  You should get a 'Repair Install' option if you want to overwrite a module.  Part of the decision process in the program logic is the comparison of controller names.  If there's a mismatch somewhere, DNN thinks it's installing a new module.  So it will continue on that path and tell you 'wait a minute, there's a key in the database already with this name.'
   Should this happen, compare the Extension properties on your local site with the server site.  In my case the server site is on Azure:
   
   
   
   
   Further down the Extension Settings page, find the Module Definitions:
   
   
   Make sure the names match in these dialogs between the local and server.
2. SQL On-Premises errors being thrown when importing a .bacpac file via SSMS:
   Very important -- the issue has been addressed by Microsoft.  You need to be running version 17.6 of SSMS or later.  On the Download screen there is a notice about maintenance programs.  If you're running these, use 17.5.  I'm not sure that this Import issue is fixed in 17.5, just assuming it is.
   Now that all the legal stuff is out of the way ...
   a. Could not import package.
   Warning SQL72012: The object [ISSIDev_Copy_VIA_SSSM_AZURE_Data] exists in the target, but it will not be dropped even though you selected the 'Generate drop statements for objects that are in the target database but that are not in the source' check box. 
   b. Error SQL72014: .Net SqlClient Data Provider: Msg 102, Level 15, State 1, Line 1 Incorrect syntax near 'EXTERNAL'.
   c. Error SQL72045: Script execution error.  The executed script:CREATE EXTERNAL DATA SOURCE [myDataSource] 
   WITH (
   TYPE = BLOB_STORAGE,
   LOCATION = N'https://dtstestcsv.blob.core.windows.net/sunsource',
   CREDENTIAL = [AzureStorageCredential]
   );
   The fix is a bit detailed.  I thank Sagarika Kondabolu from Microsoft for writing the following.  Note, it took a couple of hours or maybe 3 to resolve this.  She worked through lunch and is commended for doing that. :)  She was seated next to the rep who solved the issue so they collaborated and we all got this one solved.  He'd overheard the issue and helped out.  So thanks very much to you both from me.
   
   

   Hi Michael, 

   No problem, thanks for your time too. Below details helps you to post the issue in forums, Feel free to let me know if you need any further details. 

   Symptoms: 

   When using blob storage auditing or threat detection on Azure SQL Server or Database (or) TDE enabled on the database; after you export database to .bacpac file,

   if you import the database to local on-premise SQL Server installation you may get either of following errors (or) both of them: 

   Error 1

   Error SQL72014: .Net SqlClient Data Provider: Msg 102, Level 15, State 1, Line 1 Incorrect syntax near 'EXTERNAL'.

   Error SQL72045: Script execution error.  The executed script:

   CREATE EXTERNAL DATA SOURCE [xxxxxxx]

       WITH (

       TYPE = BLOB_STORAGE

       LOCATION = N'https://xxxxxxxxxxx',

       CREDENTIAL = [xxxxxxx]

       ); 

   • Drop the ‘External Data Source’ using below script 

   DROP EXTERNAL DATA SOURCE [myDataSource] 
   
   My edit here -- the name really is [myDataSource] -- just run this script.

   Error 2

   Error SQL72045: Script execution error.  The executed script:

   CREATE MASTER KEY;

   Resolution:

   • Turn OFF auditing and threat detection on the server and on the database
   • Turn OFF TDE encryption on the database (Author Edit -- Transparent Data Encryption ... no wonder I didn't see anything happening when it ran ... :)  Just seeing who's paying attention )
     Then export bacpac file and import it
     An Author Edit here -- this will help you to find the Settings.  Select your database, go to Settings and you'll find these items.
     
     
   • Update SSMS to 17.6 and above versions. It has fixes for all ‘Master Key’ related issues -- see above.

   (or)

   • Create password for Master Key and then export bacpac file and import it

        ALTER MASTER KEY ADD ENCRYPTION BY PASSWORD = '<PasswordHere>'; 

   Cause: 

   This caused by a different behavior between Azure SQL DB and Local SQL Server installation. A master key without password is an Azure SQL DB only feature, while local SQL Server installation must have password encryption for master key.

   Scripts to use 

   • Drop External Data Source 

   DROP EXTERNAL DATA SOURCE [myDataSource] 

   • Find Master Keys 

   SELECT d.is_master_key_encrypted_by_server

   FROM sys.databases AS d

   WHERE d.name = '[TestDatabase]'; 

   • Drop Master Key
     
     DROP MASTER KEY;

   
   Create password for Master Key 
   ALTER MASTER KEY ADD ENCRYPTION BY PASSWORD = '<PasswordHere>';

Took some time out of a very busy day when I have to push this site out but it's worth it to give back.

Happy DNN'ing,

Michael Durthaler