Depends on where it's leaking. Is water leaking into the bowl, or out of the base on the floor? If the floor, you need a new wax ring. If into the bowl, maybe wrong size flapper or it needs to be adjusted.
My back door is close to a roof valley. I have gutters, but sometimes rain comes down so fast that it just flows over the gutter and the barrier that sticks up. Then...
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There are a couple of options to check out-https://www.familyhandyman.com/plumbing/toilet-repair/how-to-stop-a-running-toilet/view-all/
Water level may be to high and draining down the overflow pipe. Adjust the water level lower.