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The Flash -- "Duet" -- FLA317a_0178b.jpg -- Pictured (L-R): Carlos Valdes as Cisco Ramon, Grant Gustin as Barry Allen, Candice Patton as Iris West, Tom Cavanagh as Harrison Wells, Chris Wood as Mike, Melissa Benoist as Kara/Supergirl, Danielle Panabaker as Caitlin Snow, and David Harewood as Hank Henshaw -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0521b.jpg -- Pictured (L-R): Darren Criss as Music Meister and Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0912b.jpg -- Pictured (L-R): Melissa Benoist as Kara, Grant Gustin as Barry Allen and Darren Criss as Music Meister -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved

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The Flash -- "Duet" -- FLA317b_1080b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen and Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1109b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen and Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_BTS_1414b.jpg -- Pictured: Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0544b.jpg -- Pictured (L-R): Candice Patton as Iris West, Victor Garber as Professor Martin Stein and Jesse L. Martin as Detective Joe West -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0081b.jpg -- Pictured (L-R): Carlos Valdes as Cisco Ramon, Tom Cavanagh as Harrison Wells, Grant Gustin as Barry Allen and Keiynan Lonsdale as Wally West -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0147b.jpg -- Pictured (L-R): Chris Wood as Mike, Melissa Benoist as Kara/Supergirl and David Harewood as Hank Henshaw -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0178b.jpg -- Pictured (L-R): Carlos Valdes as Cisco Ramon, Grant Gustin as Barry Allen, Candice Patton as Iris West, Tom Cavanagh as Harrison Wells, Chris Wood as Mike, Melissa Benoist as Kara/Supergirl, Danielle Panabaker as Caitlin Snow, and David Harewood as Hank Henshaw -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0237b.jpg -- Pictured: Carlos Valdes as Cisco Ramon -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0297b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen, Melissa Benoist as Kara/Supergirl and Chris Wood as Mike -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved

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The Flash -- "Duet" -- FLA317a_0336b.jpg -- Pictured (L-R): David Harewood as Hank Henshaw, Candice Patton as Iris West, Grant Gustin as Barry Allen, Melissa Benoist as Kara, Chris Wood as Mike and Carlos Valdes as Cisco Ramon -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317a_0386b.jpg -- Pictured (L-R): Carlos Valdes as Cisco Ramon and Chris Wood as Mike -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0012b.jpg -- Pictured (L-R): Darren Criss as Music Meister and Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0026b.jpg -- Pictured: John Barrowman as Malcolm Merlyn -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0107b.jpg -- Pictured: Carlos Valdes as Cisco Ramon -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0212b.jpg -- Pictured (L-R): Darren Criss as Music Meister, Jeremy Jordan as Winn Schott and Carlos Valdes as Cisco Ramon -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0252b.jpg -- Pictured: Carlos Valdes as Cisco Ramon -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0348b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen, Melissa Benoist as Kara, Carlos Valdes as Cisco Ramon, Darren Criss as Music Meister and John Barrowman as Malcolm Merlyn -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0381b.jpg -- Pictured (L-R): Jeremy Jordan as Winn Schott and Darren Criss as Music Meister -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0405b.jpg -- Pictured (L-R): Darren Criss as Music Meister, Jeremy Jordan as Winn Schott and Carlos Valdes as Cisco Ramon -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0521b.jpg -- Pictured (L-R): Darren Criss as Music Meister and Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0600b.jpg -- Pictured (L-R): Melissa Benoist as Kara and Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0654b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen, Melissa Benoist as Kara, Carlos Valdes as Cisco Ramon, Darren Criss as Music Meister and John Barrowman as Malcolm Merlyn -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_0912b.jpg -- Pictured (L-R): Melissa Benoist as Kara, Grant Gustin as Barry Allen and Darren Criss as Music Meister -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved

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The Flash -- "Duet" -- FLA317b_1053b.jpg -- Pictured (L-R): Melissa Benoist as Kara and Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1080b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen and Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1102b.jpg -- Pictured (L-R): Melissa Benoist as Kara and Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1109b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen and Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1298b.jpg -- Pictured (L-R): Melissa Benoist as Kara and Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_1327b.jpg -- Pictured: Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317b_BTS_1414b.jpg -- Pictured (L-R): Jeremy Jordan as Winn Schott -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0036b.jpg -- Pictured (L-R): Victor Garber as Professor Martin Stein, Jesse L. Martin as Detective Joe West, Grant Gustin as Barry Allen and Melissa Benoist as Kara-- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0045b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen and Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0049b.jpg -- Pictured: Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0080b.jpg -- Pictured: Jesse L. Martin as Detective Joe West -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0139b.jpg -- Pictured: Melissa Benoist as Kara -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0205b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen, Candice Patton as Iris West, Jesse L. Martin as Detective Joe West and Victor Garber as Professor Martin Stein -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0229b.jpg -- Pictured: Grant Gustin as Barry Allen -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0297b.jpg -- Pictured (L-R): Grant Gustin as Barry Allen, Jesse L. Martin as Detective Joe West, Candice Patton as Iris West and Victor Garber as Professor Martin Stein -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0318b.jpg -- Pictured (L-R): Candice Patton as Iris West, Jesse L. Martin as Detective Joe West and Victor Garber as Professor Martin Stein -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0464b.jpg -- Pictured (L-R): Victor Garber as Professor Martin Stein and Jesse L. Martin as Detective Joe West -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317c_0544b.jpg -- Pictured (L-R): Candice Patton as Iris West, Victor Garber as Professor Martin Stein and Jesse L. Martin as Detective Joe West -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All rights reserved.

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The Flash -- "Duet" -- FLA317d_0012.jpg -- Pictured (L-R): Melissa Benoist as Kara and Grant Gustin as Barry Allen -- Photo: The CW -- © 2017 The CW Network, LLC. All rights reserved.