2008 election as MP

On January 19, 2007, Duncan accepted the NDP nomination in Edmonton—Strathcona, by acclamation, to again seek election to Parliament in the 40th Canadian federal election.^[6] The election campaign began in September 2008. To make environmental protection an election issue, Duncan and Jack Layton flew over the oil sands area noting environmental impact.^[7] Duncan made support for public health care, enforcement of environmental laws, and driving the economy with 'green jobs' priorities in her campaign.^[8] She drew upon support from a large volunteer network built since the last election and strategic voting from Liberal supporters.^[9] With the polls showing a close race, Jaffer launched an attack ad against Duncan.^[10]

On election night, October 14, the results showed Jaffer as the leader, by 1,000 votes with over half the polls reporting.^[11] Jaffer delivered his victory speech around 10 p.m. and several people had projected Jaffer had been reelected.^[12][13][14] However, late polls, which included residences around the University of Alberta, put Duncan ahead.^[15] Following a few days of silence and after his fiancée, fellow Conservative Member of Parliament Helena Guergis, flew to Edmonton and quietly married him, Jaffer conceded defeat to Duncan.^[16] With a 463-vote margin, Duncan became the only non-Conservative MP in Alberta, and the first NDP member from the province since 1988.

Linda Duncan at the Alberta Legislature participating in a rally organized by RETA against the Heartland Transmission project

During the ensuing 40th Canadian Parliament Duncan was promoted to the NDP front bench as critic for the environment. In the Parliament's aborted first session she strongly opposed the government's proposed fiscal update, especially the proposed changes to pay equity claims, four-year wage cap, and suspension of the right to strike for federal employees.^[17] She supported the proposed coalition government and condemned the Prime Minister, Stephen Harper, for labeling the coalition government as "treasonous" and "criminal".^[18][19] When Parliament resumed in January 2009, Duncan sat as a member of the House Standing Committee on Environment and Sustainable Development. She supported Ecojustice and the Sierra Club's lawsuit against the government's waiver of federal environmental assessment reviews on infrastructure projects arguing that it required an act of Parliament, rather than the Conservative government's Order in Council.^[20] She vocally supported the opposition's Corporate Accountability of Mining, Oil and Gas Corporations in Developing Countries Act that would hold Canadian companies accountable in Canadian courts for human rights and environmental abuses committed in other countries.^[21] Duncan introduced three bills into during the second and third sessions: the first proposing that the third Friday of February be declared National Hockey Day,^[22] the second establishing an Environmental Bill of Rights,^[23] and the third would have amended the Criminal Code to restrict the use of hand-held telecommunications devices while driving.^[24] On a private member's bill to abolish the federal gun registry, Linda Duncan was the only MP from Alberta who voted against abolishing the gun registry.

2011 re-election

Duncan was re-elected, with over 50% of the vote in Edmonton—Strathcona, to the 41st Canadian Parliament. She introduced one piece of legislation, a private member bill titled National Literacy Policy Act (Bill C-327) which received first reading on October 5, 2011, but did not advance from there. The bill would have required the government adopt a policy for promoting literacy in Canada and take measures to enact the policy.

OSCE Parliamentary Assembly Mission Canadian M.P. Mark Warawa and Linda Duncan fill out Observation forms at a Polling Station in Lviv on 2014 Ukrainian Parliamentary Election Day

In 2014, Linda Duncan introduced an Act to establish a Canadian Environmental Bill of Rights, Bill C-634, "whose provisions apply to all decisions that emanate from a federal source or are related to federal land or a federal work or undertaking".^[25]

Duncan spoke at a Jack Layton memorial on August 24, 2012. The event was billed as "Dear Jack" and she was joined by several other prominent figures.^[26]

2015 re-election and final term

Duncan was re-elected for a third term in the 2015 federal election. She was appointed the NDP critic for Transport in the 42nd Canadian Parliament.^[27]

In August 2018, Duncan announced that she would not seek re-election in the 2019 federal election.^[28]