Drag pour sounds like trailing. It's basically when a measuring tool such as a jigger is used, but the bartender 'trails' a little extra liquor into the glass as he pours the jigger into the glass.
Never heard of double dipping, unless they're talking about when the bartender free pours a liquor into a glass by tilting it over, tilts the bottle upright, then tilts it over to continue the pour.
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