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Let's not forget they're making TWO series (4 & 5) this year. The previous series were all done once a year, were they not?
Series 1 came out in April 2003 and Series 2 came out in November 2003. I think that with the long wait for Series 3 (October 2004) most people thought that they had came out once a year. We did get the brilliant Speeder Bike box set, carded droids figures, and the carded Sandtrooper in between that long wait though.