Hi Faizal
If you first select and begin to drag a clip and then hold down the Option Key you can perform a Swap Edit. (Note that the cursor will change accordingly to show a swap edit will be performed).
If you already hold down the option before you perform the select and drag move, and keep the option key depressed whilst placing the clip, then you will perform an Insert Edit with a duplicate of the selected clip. (Note the cursor will show that an Insert Edit will be performed).
Or again as above, if you already hold down the option before you perform the select and drag move and then release the option key before placing the clip, then you will perform an Overwrite Edit with a duplicate of the selected clip. (Again note that the cursor will show that an Overwrite Edit will be performed).
Hope it helps
Andy