Master He is on duty every 3 days, and Master Chen is on duty every 4 days. They are on duty at the same time on June 1. How many times are they on duty at the same time this month?
何师傅每隔3天值一次班，陈师傅每隔4天值一次班，6月1日他们同时值班，这个月他们有几次同时值班？
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