what to fill my wood mistake with? something durable, carpenters chime in

[darkone]

Hey all, we buggered up my 2 outside boards and put a quarter inch groove the length of the board in the wrong spot. We flipped it over and now this groove will be on the backside hidden. I feel like this groove should be filled with something though for the added stregth? What would you recommend, and at what stage should I add the said filler. After sealer? Before? The wood is mahogany. Any help would be greatly appreciated. Thanks in advance.
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