There is a basic rule of thumb in the Windows environment. Dragging something will move it; dragging while holding down the control key will create a copy. Unbeknown to many, this simple principle works across most applications, including Excel and is easy to use when trying to duplicate a worksheet.
If I use my mouse to drag a sheet tab, it just changes its position. However, if I hold down the control key at the same time, a little plus sign appears, indicating that a duplicate worksheet will be created.
The same principle can be employed across Workbooks. Dragging a tab from one Workbook to another will move it. Holding down the control key will place a copy in your second Workbook.