A good duplex can provide you with private yard, an attached garage, and more of a "house" feeling but renting out the other half can dramatically reduce your expenses and possibly even allow you to live there for free.

That dramatically reduced the humidity in the basement.

If you upgrade the exterior dramatically, the building may end up standing out and looking odd / not fitting in...

The reason I ask a while ago I think I heard something about being able to "bundle" your properties together to reduce the price of utilities dramatically.

That will dramatically change what financing abilities you have at your disposal. 

Just be aware that many people are using the same strategy and as a result the prices of small multifamily properties have dramatically increased over the last few years. 

Great - but .... from one neighborhood to the next the values can vary dramatically so you need to know your comps....

In my market (Columbus, OH) a SF next door to a duplex unit isn't going to rent for dramatically more in most areas, assuming similar factors of condition, sq ft, etc..

Once you ross one values/rents can shift dramatically.

This will dramatically raise your NOI and you can then use that for accelerated savings for your next property.