“Id like to take my duplex and add funds from my 1031 exchange so that I can turn it into a fourplex. Can I do a 1031 exchange into an investment property I own without paying taxes?”
***ANSWER:
Basically the answer is NO, you cannot exchange into a property you already own.
However
If you hire top-notch, expensive tax attorneys and go through complex and expensive legal maneuvering, you may be able to legally create other entities that allow you to accomplish that goal. But this is typically done only in very large transactions that justify the large legal expense and time-consuming process.
Please see your tax professional.
More Info On 1031 Exchange