First, hold the CTRL button on your keyboard.

Next, while holding the CTRL button, click on the sheet you want to copy, drag it toward the right side, and let go of the left mouse button.

This will create a copy of the sheet.

In the menu that appears, scroll up and select the ‘Move or Copy…' option.

Then, in the pop up selection box, choose the workbook you want to paste your duplicate sheet into (or start a new book). Once it is selected and highlighted blue, click ‘OK'.

Then, select the sheet you want to appear before your duplicate sheet. Once that option is highlighted blue.

Click the box below with ‘Create a copy' written beside it – do not click ‘OK' until this box has been ticked otherwise you will just be moving your spreadsheet instead of duplicating it. Once you have, you can then click ‘OK'.

This will duplicate the sheet and purposefully place it next to your desired sheet in your desired workbook.

You will instead need to hold down the ‘Option' button which can be found between the ‘Control' and ‘Command' button on your Mac keyboard.

While holding down the ‘Option' button, you can click and drag the sheet tab to create a duplicate. Then, drop the sheet tab down and release your ‘Option' button.

If you want to move your duplicate sheet into a different workbook, then you will have to open up both workbooks (one with the original, the other one is the one you want to move your duplicate sheet to).

Right click on the original sheet tab and scroll up the menu to ‘Move or Copy…' 

In the options box that appears, select the workbook where you want to move your duplicate sheet to and once it is highlighted green, tick the ‘Create a copy' box underneath then select the ‘OK' button.