How many years has a nation participated in the Eurovision Song Contest with more than one entry?

Explanation

In the very first year of the Eurovision Song Contest, in 1957, 7 of the founding members of the EBU (European Broadcasting Union) participated with 2 songs each. Denmark, Austrian and Great Britain signed up too late and had to wait for the next year. This is the only year where nations have been able to send more than one country. The first contest was by by Lys Assia from Switzerland, who performed the song "Refrain".