Lithuanian, Latvian or Estonian citizenship in 1951 had lost their Polish citizenship (see clause 4 of the second law). Nonetheless, if the ex-Pole returned to Poland afterwards due to the different agreements of repatriation that were signed between the USSR and Poland (e.g. in 1945 and 1956), then they regained their Polish citizenship. In fact, every Pole that became a Soviet citizen and did not take advantage of the opportunity to come back to Poland due to these agreements, lost their Polish citizenship. In order to confirm the citizenship of a person who left before 1951, it will be easier to prove that they left Poland after the first law from 1920 entered into force. Otherwise it will be difficult to prove their Polish citizenship. If the parents stayed in Poland after 1920 it might help. The law from 1920 allows for citizenship to pass from father to his born-out-of-wedlock child only if the father declared his paternity before the child turned 18 and only in front of Polish authorities. Hence, without an original birth certificate with the father's name it might be difficult to prove. The law from 1962 allows for citizenship to pass from father to his born-out-of-wedlock child only if the father declared his paternity within one year from birth. Hence, such children of only a Polish father (mother is not Polish) born when this law was in force must show an original birth certificate or a paternity declaration signed before they turned one year old (in Israel, this declaration is usually done in the hospital, when registering as the newborn's father and it is saved in the archives of the Ministry of Interior and a copy of it can be issued on request). The issue of citizenship of children whose parents had different nationalities was regulated not only in the provisions of the Act on Polish Citizenship, but also in the international agreements ratified by Poland in the field of citizenship. This means that, in such a case, the provisions of the Polish Citizenship Act were not applied. In the 60s and the 70s Poland signed with some countries of the Central and Eastern Europe the conventions on avoidance of multiple nationality from which withdrew in the 90s and 2000s. Dual citizenship Polish law does not explicitly allow dual citizenship, but possession of another citizenship is tolerated since there are no penalties for its possession alone.