Without getting into a lot of down-stream issues, the ones that I faced on this very question boil down to 2 key issues.
1. Will your employer allow you to wire or wirelessly sync your smart phone into his computer system on which you have your work Outlook account? (I have just made a lot of assumptions, one of which is that you work for someone else and that you use a similar setup that I do, namely smart phone and Outlook.)
2. Are you willing to live with your employer's likely requirements that Outlook (and probably your smart phone) are subject to inspection or even confiscation at any time if he wants to exercise full control over his data that resides in the same places as your personal data?
In my case, the answer to both questions is a resounding "no," so I keep my systems as separate as I can. My work day is pretty much during the day on weekdays, so my smart phone calendar has very little on it for those hours. About the only time that I have to show duplicate appointments is when I have to leave work for a personal appointment. I need to show that on my work calendar so that I (and anyone looking at my work calendar) know that I have to be out of the office.
The only other "work thing" that I can think of that is on my smart phone is an encrypted set of passwords to my various work accounts (along with all of my personal ones) in an Android app.
In spite of all of this "separateness," there is almost no duplication. I always cite the old saying: "A man with one watch knows what time it is; a man with two is never sure." So, each data record resides in only one place (that is backed up somewhere, via sync'ing or otherwise), but the whole set of data happens to reside in 2 places, namely work and personal devices.
I hope this helps.