This stamp comes from Georgia, circa 1921 I believe.
I'm not sure if this is very common for the area and time.
I believe what happened is that, prior to the Russian revolution, the stamp was valued at 20 rubles. (Georgia had a relation to Russia, which was its protectorate.).
Afterwards, when the USSR was formed, hyper-inflation occurred. With poverty abound, they took existing sheets, and printed on the reverse new stamps (similar to Germany's million-dollar stamps).
Eventually, they devalued the existing currency, similar to what Russia did in the 1990s. Thus, it ended up the the 5 kopek overstamp.
Is this unusual, or pretty common for the area and time? I paid a bit for it, but I suppose that doesn't mean too much, and am wondering if I should pay to get it authenticated, if it has any value?