VGFacts

A portion of the Mario Kart 7 Bowser's Castle theme is a remix of a part of Mario Kart: Double Dash's theme.

Here is the Mario Kart 7 Bowser's Castle theme:


Here is Mario Kart: Double Dash's Bowser Castle's theme:


Here is also a link that states the trivia:
http://www.mariowiki.com/Bowser%27s_Castle_(3DS)