To answer your question of if you should refinance an adjustable rate mortgage on an investment property, my preliminary recommendation is yes, you should refinance. Although mortgage rates are always unpredictable, the Federal Reserve's outlook as well as general probability suggest that rates are more likely to go up than down over the course of the next year. Although there is no guarantee which direction mortgage rates will move, I recommend that you consider refinancing into a 20 year fixed rate mortgage while interest rates continue to be relatively low. You can use our Refinance Calculator to understand the financial impact of refinancing and review our helpful overview of reasons to refinance your mortgage.
Your other option is to refinance into a 30 year fixed rate mortgagee but overpay and make the same monthly payment that you would with a 20 year loan. That way you benefit from mortgage acceleration while maintaining the flexibility to make the lower payment required for a 30 year mortgage, if your financial situation calls for it. Either way, refinancing into a 20 or 30 year fixed rate mortgage at today's relatively low rates eliminates the risk that the mortgage rate and monthly payment on your current adjustable rate mortgage (ARM) increase in the future.
Finally, we always recommend that you shop multiple lenders to find the loan with the best terms. You can review non-owner occupied lenders in your area by clicking INTEREST RATES. Use the "Refine Your Search" menu to adjust for Home Type (2 unit - duplex) and Loan Type and Term. We advise you to contact at least four lenders as comparing lenders is the best way to save money on your mortgage.