Wednesday's announcement that Lubbock Power & Light will acquire Xcel Energy's in-city customers brings life to the city's downtown redevelopment hopes.  The doubled power lines over downtown's streets -- some belonging to LP&L, others to Xcel -- has been the lone impediment to getting redevelopment from blueprints to reality.   Under its agreement with master developer The McDougal Companies, the city is responsible for moving the lines.   The twinned lines have existed since 1942.    "It's the oldest part of the city and the oldest part of the infrastructure," Mayor Tom Martin said. "It would be expensive to duplicate two sets of infrastructure."   "Without this, downtown redevelopment doesn't happen," said former Mayor Marc McDougal, who represented the McDougal Companies at a breakfast for key city business officials to be briefed on the move yesterday.    The McDougal Companies were retained in December on a $1.7 million contract as master developer for downtown revitalization.