If you pray the rosary multiple times a day do you pray the same mystery several times a day?
If you like. Or you can pray all five mysteries. The only thing I wouldn't do (and this is personal preference, along with guidelines--not hard rules--from the USCCB) is pray the Joyful Mysteries during Advent and Lent, except on Sundays and Solemnities.
You can't pray all the mysteries in one day can you?
Sure you can! Most people pray an entire Rosary (one complete set of Mysteries) in about 20 minutes. You could even space it through the day: your own "liturgy of the hours." First thing in the morning, midday, 3pm (Sorrowful Mysteries), evening, and bedtime. You can try praying all the mysteries at one time if you want, but you're likely to wear yourself out doing so. Praying the Rosary is a very active prayer. Your hands, lips, and mind are all working at the same time.