Interesting, but odd. I would have thought it was more like putting down two chips on red rather than one. After all, you can have two life insurance polices, both of which pay out when you die. But not two health insurance policies that pay out of you're sick. I wonder what the distinction is, and how they draw it.
In some cases you'd be completely correct... and it's allowed in some cases. DJI's plan is not insurance, it's a warranty. So you could collect from that all day and every day. However, an Inland Marine policy is going to contain a clause that _should_ not allow you to collect from it and another source.
But let me correct myself a bit... I don't know for sure how any one policy is written. An insurance company that uses a standard ISO form policy (such as State Farm) _should_ have this clause. But there is another "however". It might also state something like, "other insurance". If so, you _might_ be able to argue that DJI's plan is not insurance. But at that point you are looking to trip up the policy language in your favor.
Could you collect twice? Yes. Is it likely? No.
Health plans are different and there are some state laws that work like you mention. They consider you are paying for twice the coverage, you should be able to collect twice. But this is state and case law that addresses that.