Not including remixes.... (Those probably don't count)
Vulnerable Battlefield, The Gods Give Us Blessed Rain, and both versions of Player's Score sound very similar (the eighth note section)
UN Owen, Heian Alien, Lonesome Werewolf similar (obvious)
Maple Wise and Legendary Illusion are similar in one section
Maple Wise and Solar Sect of Mystic Wisdom have almost the same chord progression. You could mix those two together easily.
Cheat Against Impossible Danmaku and Alice in Wonderland have a similar melody
Alice Maestra and Nostalgia Blood of the East
Romantic Children and Nostalgia Blood of the East
Plastic Mind and Nostalgia Blood of the East