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It is super easy to implement, though. Just store the isGrounded state with a float instead of a bool, set it to a small value like 0.1 seconds when you touch ground, make it reduce by time, and whenever the player press the jump button, it checks if the value is greater than 0. If yes, set the value to 0 and do the jump function.

If you are using Unity or C# and you already have multiple functions accessing the isGrounded bool, you can turn it into a "property with custom accessors" (a property with a custom getter and a custom setter).