I just finished completely relacing my rear wheel for the exact reason you stated here. Usually once spokes (more than one) start breaking in succession, they don't stop until until you have replaced them all. Metal fatique is pretty equal in the spokes of a well tensioned wheel. It's weakest link syndrome.
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2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153
*estimated