I'd suggest to experiment a bit - you can make a new template with just one DCI and see what happens when you export or import that.
For duplicates - it's controlled by GUID of each template that is included in the .xml file. So if you import it back, it will replace the original template, even if the name is different. But if you change the GUIDs in the .xml, it will import as a new one.
Only "Replace templates" option is relevant if your .xml contains only templates. If it's not checked, then if template already exists, no changes will be done to it.
For duplicates - it's controlled by GUID of each template that is included in the .xml file. So if you import it back, it will replace the original template, even if the name is different. But if you change the GUIDs in the .xml, it will import as a new one.
Only "Replace templates" option is relevant if your .xml contains only templates. If it's not checked, then if template already exists, no changes will be done to it.