Video download after editing is too long.

Can anyone answer this:

I put together a short video -- about 5 minutes. But then I "exported and shared" it to my desktop (as an mp4 video), the result was a 10-minute piece. The 1st 5 minutes were the video I created. The rest of the time was just plain black. Does anyone know how I can "save and export" only the lengthof the video I created, without the extra time being added? 

Please respond, if you can, toi my email:   

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Thanks,

-- Ben Martin