Timing is a key factor

One of the important principles of using keyboard modifiers is that the same key will have different effects, depending on when you use it. For example, with the Direct Selection tool (A) active, holding down the Option key (PC: Alt key; MAC: Option key) will select the entire path. If you don't let go of the Option key while you drag and move, you'll get duplicate. So, to move a path without duplicating it, hold the Option key while pressing and click to select the path, release the Alt/Option key before dragging the path and move.