Recently I was asked about the difference between the duplicate and copy commands, since they seemingly do the same thing.
Duplicate (Ctrl-D) will automatically copy and paste whatever slide you select and place it right below the original slide. It will be identical in every way to the previous slide.
Copy, (Ctrl-C) will save a copy of the slide to your clipboard so that it can be pasted (Ctrl-V) elsewhere in the presentation.
Choosing duplicate is a time saver because it negates having to manually paste the slide by doing it automatically for you. However, if you would like to put the duplicate slide somewhere in the presentation other than right after its original, you will want to use the Copy and Paste technique.
You will also want to use the Copy command if you would like to paste the slide into another application (like Microsoft Word, for example).
Very important! Keep in mind that if you copy the slide and then paste it into another PowerPoint presentation, the pasted slide will take on the slide theme and formatting of whatever slide is immediately before it, even if it is different from the original's theme.