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Look what Blake Lively made Taylor Swift do ... the pop star is set to give evidence in her former best friend's legal war with Justin Baldoni, TMZ has learned.

In new court documents obtained by TMZ ... it confirms the singer is set to give a deposition later in October, and that the selected dates worked with T Swift's "preexisting professional obligations" -- likely referring to the upcoming press tour tied to her 12th studio album, "The Life of a Showgirl." Getty

Per the docs, Baldoni's camp asks the court to modify the scheduling order to accommodate Tay's deposition ... noting the Grammy winner is not able to testify before October 20th.

Remember, Swift got dragged into the Lively-Baldoni "It Ends With Us" drama when the actress allegedly referred to the singer as one of

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