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In the Schengen visa form, you only enter the dates for the first trip that you will make into the Schengen Zone. Information about subsequent trips can be included in the itinerary that you also have to submit as a part of your application. You may also include a letter explaining why you wish to enter the Schengen Zone multiple times.
However, keep in mind that despite all this, you may only receive a single entry visa. At the same time, many consulates may also issue a multiple entry visa even when you only indicate that you are planning to visit once.
Source: personal experience with applying for Schengen visas multiple times.