myUniqueList = []
myLeftovers = []

def addList(newThing):
     if newThing in myUniqueList:
         myLeftovers.append(newThing)
         return False
     else:
         myUniqueList.append(newThing)
         return True

print(myUniqueList) # []
print(addList("Meliodas")) # returns 'True' since it's a new item
print(addList("Escanor")) # returns 'True' since it's a new item
print(addList("Meliodas")) # returns 'False' since it's already been added
print(myUniqueList) # This includes the new entries
print(myLeftovers) # This includes any repeated entries