@Len Kaufman Usually clips get duplicated if you Ctrl + drag. Is your finger anywhere near your keyboard Ctrl? That's the only cause I can think of.
@EricLNZ Thank you, Eric. I'll try to be more conscious of that. I'll keep you posted if that's the thing. I hope it's as simple as that.
In Vegas 23, I cannot find a way to swap the timeline with the docking area. In other words, put the timeline on the top, which I have been doing for decades. There's always been a checkbox for this in Options | Preferences, usually on the Display panel.
@EricLNZ Though I can't try your suggestion just now, as my computer is rendering out a lengthy video, I did some digging, i.e. Google, which is surprisingly good at finding out things about VP. It confirms what you said, but there is a "condition" which can change the function. It said ctrl+drag if you place the cursor on the edge of the clip speeds up a clip. And if you place the cursor in the body of the clip, it will create a dupe. Never heard that one before. When the render is finished, I will experiment. Came across another option which I, and likely others, will find useful, and that is described here: 
In that technique, you create a box around the entire area of clips, including multiple timelines, and that will allow you to drag a bunch of clips. I'm learning something new every day. There is so much depth to VP, in all its iterations. I often marvel at the complexity of it, and the minds that went into putting it together.