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You have a problem. According to the page on the Russian Embassy in Ulaanbaatar in order for you to obtain a Russian visa there:
To apply for any category of visas except transit, one must have a long-term registration in Mongolia (βAlien registration cardβ valid for 1 year).
There are exceptions in the following 2 paragraphs for various countries that do not require to have a registration but Germany unfortunately isn't one of them.
So yes you may be able to obtain another visa following the one you already have just not in Mongolia.