There’s no reason other than history. There’s a pattern for multiples: triple, quadruple, quintuple. But there are also words like couple, example, trample, steeple. Alongside of apple, grapple, popple, supple, etc.
For any particular word, basic etymologies can be found at [etymonline.com](https://etymonline.com). But I don’t think there’s any clear pattern. It’s just the accidents of history tht got frozen into “standard” spellings.
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