Why is my BRAND NEW car battery constantly dying?

We have a 2002 Honda Civic. Mileage is good but we were having problems with the car battery constantly dying and so we replaced the original one with a brand new one around 6 weeks ago. The new one has died on us twice so far, once was the week after we put in the new one but it was because we forgot the GPS plugged in, which our guess is that was the cause. We were able to get that issue by having a kind neighbor jump start us. The second time was only just today and this time around we had nothing plugged in or left on, so we're trying to make sense of what it is that causes a brand new battery to die and to see what the solutions may be.

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