Can You Duplicate Things?

I have no idea if the iD editor can do that but it's trivially easy in JOSM. My advice to anybody who is now or might become a major contributor to OSM is to start using JOSM as soon as possible.

In JOSM, first select the object (node, way or whatever you've selected) by clicking on it. Move your mouse to the desired position of the new object, and press [Ctr]-D. Done.

If your initial placement wasn't perfect you can select the new object and move it by holding down the [Shift] key and striking one of the arrow keys as many times as are needed to position it. The object can also be scaled and/or rotated by using a couple of JOSM's keyboard shortcuts. While holding [Ctrl] and [Shift] move the mouse pointer up or down to rotate it. Holding both [Ctrl] and [Alt] allows resizing. Moving the mouse pointer up makes it smaller, moving the mouse pointer down makes it larger.

Try JOSM. You'll like it. The only sticking point for people jumping from other editors is that to pan the imagery in the edit window, you click and hold the right mouse button while dragging. Most other maps including Bing and Google Maps use the left-button for this purpose.

Hope this helps