Because each tab in the same file needs to have a unique name, Excel will add a number at the end in parenthesis like 2013 (2) whenever you copy a spreadsheet in the same workbook.
If the tabs in your workbook have numbers in them, you can have that number automatically increment by +1 when you create a new tab by copying an existing tab.
Let’s say your tabs are the year numbers 2010, 2011, 2012, 2013, and 2014. When you want to create a new tab for 2015 you would need to copy the 2014 tab, and edit the new 2014 (2) tab name to 2015. Instead of using just the year number, or whatever number you are using, put parenthesis around the number. Your tabs will now look like (2010), (2011), (2012), (2013), (2014).
When it comes time to copy the 2014 year’s tab to create the 2015 tab, Excel will auto increment the number in the tab to (2015). You can add any text outside the parenthesis like the month name, region, etc. If you add the text inside the parenthesis, the auto increment will not work.