Client got invoiced before the year end, but paid in the new year, how to handle that?

You will include that income in the 2015 tax return.  The fact that the client delayed paying you just means it appeared on the books as a receivable, and the receivable was paid in 2016.

Do I have to declare it again in 2016?

No you won't have to declare it again in 2016.

Awesome! Thanks 🙂