A double-bundle graft in ACL surgery uses two bundles of graft material instead of one, to try to mimic the normal ligament.

Technique of double-bundle ACL reconstruction
Double-bundle reconstruction is more challenging to perform, and from the patient's perspective there are two additional bone tunnels and one additional incision (skin cut).
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CLINICAL PUBLICATIONS
Postoperative Evaluation after Anterior Cruciate Ligament Reconstruction: Measurements and Abnormalities on Radiographic and CT Imaging. Kim M, Choi YS, Kim H and Choi N-H. Korean J Radiol. 2016 Nov-Dec; 17(6): 919–930.
Double‐bundle versus single‐bundle reconstruction for anterior cruciate ligament rupture in adults. Sumanont S, Foocharoen T, Laopaiboon M and Tiamklang T. Cochrane Database Syst Rev. 2012 Nov; 2012(11):