Did your original feed to the garage have two different 120V circuits? If so, they could be fed from two different breakers that are sitting on opposite sides of the main panel, which would put 240V between L1 and L2. If something has come adrift, or some wire is pinched somewhere, that could manifest as 240V in a single duplex outlet.
btw- when I wired my garage, I ran three conductor (Red, Blk, Wht, with bare ground) from the main panel to groups of daisy chained outlets. When installing the outlets, I break out the tab between the two brass screws. One gets the Red wire, the other gets the Blk wire, and the neutral (white screw) gets the white wire. The feed comes from a dual-ganged 240V breaker. This way every outlet in the garage/workshop has 240V in the box, even though the outlets have only 120V between the narrow prong and the wide prong. That provides the equivalent of two 20A circuits into the workshop for higher capacity. If I need 240V to run a welder, etc, I can replace a ganged 120V outlet with a four prong outlet wired for 240V in any box in the garage. When I showed this to the inspector, his eyes glazed over, but he agreed that it passes code...