When state A, in which an arbitral award was rendered, decides to annul the award in accordance with its national laws, it does not necessarily mean the award is effectively null and void. Rather, the initially prevailing party X may still be granted enforcement of the award in state B. The situation gets even worse, if a second arbitration is conducted in state A, this time rendering an award in favour of the former underlying party Y. Now party Y may in turn – successfully – seek the enforcement of the second award. This book aims to analyse the legal framework and the national case law resulting in such out-comes and gives recommendations how legal certainty may be assured – de lege lata and de lege ferenda.