Does anyone know if we can copy/duplicate a tab in a split to create an other tab?
It is not possible. You will have to create it manually.
It is not possible with UI. But, if you have good understanding on XML than you can do it by following below steps.
1. Right click on workflow and select "Edit xml source ...."
2. Look for your split activity.
3. look for XML code for existing tab, will look like this
4. Copy and paste the code and save the XML.
PS: This is not a preferred route to do it, as it will require playing with XML code and possibility to corrupt your workflow if something went wrong.