Given the condition, you just need to return the second array.

General advice:

• Get the parameters of the failed test. AFAIK every programming language here at CodeWars has some way to print something.
• Get your result.
• Re-read the kata's description. There are a lot of issues where the user missed something in the description.
• Check your result manually (if possible). There are quite a few issues where the user's result was just wrong.

C++ translation kumited. Have fun!

In a Python sandbox, this function returns the right results but I still fail some tests, I don't understand why.

def merge_arrays(first, *second):
if len(second)>0:
for i, elt in enumerate(second):
if elt not in first:
first += elt
else:
first = list(set(first))
first.sort()
return first