http://en.wikipedia.org/wiki/2_euro_coin
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Commemorative issues
Each state, allowed to issue coins, may also mint one commemorative coin each year. Only €2 coins may be used in this way (for them to be legal tender) and there is a limit on the number that can be issued."
"Eurozone-wide issues do not count as a state's one-a-year issue. Germany has begun issuing one coin a year for each of its states (the German Bundesländer series which will take it up to 2021."
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Commemorative coins issued by a single country
As a rule, euro countries may each issue only one €2 commemorative coin per year. Exceptionally, they are allowed to issue another, provided that it is a joint issuance and commemorates events of European-wide importance."