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The XFL is making a big mistake when it comes to requiring its players to stand for "The Star Spangled Banner" -- so says Jaguars star Marcedes Lewis.

We got Marcedes right after Vince McMahon announced he was resurrecting the league ... and the stud TE told us certain players (see: Colin Kaepernick) won't have anything to do with the XFL if that's their stance on free speech.

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"They may not play ball ... that might affect who you're able to get in that league," Lewis told TMZ Sports.