A May 1, 2020 report by Peter Roudik for the Law Library of Congress titled Russia: New Law Eliminates Requirement to Renounce Foreign Citizenship is a useful resource for legal issues involving dual citizens of Tajikistan and Russia. Toward the end of the report, it notes that Russia neither provides for nor prohibits dual citizenship. Instead, it treats Russian citizens as citizens of Russia and no other country. However, “[a]n exception is made for the citizens of Tajikistan, the only country that has a relevant treaty with Russia.” The relevant treaty took effect in 1997. This is a good starting source for establishing that Tajik citizens who are naturalized as Russian citizens do not lose their Tajik citizenship as a result of Russian naturalization. The article also links to the relevant Russian-language treaty.
Resource on Tajik-Russia Dual Citizenship
Russian law neither provides for the possession of dual citizenship nor recognizes the foreign citizenship of its nationals. (Law on Citizenship art. 6.) Under this law, all individuals are treated as Russian citizens regardless of whether they have citizenship of a foreign country. An exception is made for the citizens of Tajikistan, the only country that has a relevant treaty with Russia. (Treaty Between the Russian Federation and Republic of Tajikistan on Regulating Dual Citizenship Issues, Sept. 7, 1995.) A similar treaty with Turkmenistan was terminated in 2015.