Hello there, @B Johnson. 

Thanks for joining in this conversation. I appreciate the detailed information you've shared. 

Your data file may be damaged this why the Duplicate isn't showing in the drop-down selection and the cause why the changes you've made in the invoice template reverts to the old one. 

Let's fix this by running the Verify and Rebuild Utility Tool.  

Rebuild Data:

1. Go to the File menu.
2. Choose Utilities, then select Rebuild Data.
3. Select OK on the QuickBooks Information window.
4. Follow the prompt to save a backup.
5. When you see Rebuild has completed, click OK.

Verify Data:

1. Go to the File menu.
2. Choose Utilities, then select Verify Data.
3. If you see QuickBooks detected no problem with your data, click OK. You can continue using the company file.
4. If Verify finds an issue with your data, click Rebuild Now or View Errors.
5. Once done fixing the data, select Close.

Check out the following article for more detailed information: Resolve data damage on your company file. 

For future reference, you may check this article: Use and customize form templates. It also includes steps to importing and exporting customized templates in QuickBooks Desktop. 

Please let me know if you have other concerns. I'm always around to help.