Not a technique I use myself, but I just tested it. If you click on the clip and then press OPTION, it won't duplicate. Hold down OPTION first, then click and drag the clip you want to duplicate. Once you're at the new location, also hold down SHIFT to change from Insert to Overwrite, then release the mouse button.
Bit of an octopus-finger operation to me, so I usually just Copy, move the original with OPTION-Up/Down Arrow, and Paste. Or, double-click to open the clip in question and then drag it right back into the timeline. More operations, but for me it's faster than doing the wrong move and then having to undo it.
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