The only part I am not sure about if the market will "correct"; I believe it is much more likely that it will level off.The reason I believe that is affordability - the NAR publishes an affordability index; basically how much of a median household incoem is needed to buy a median home with 10% down.

I am a design engineer by day, and I'm looking to diversify my index fund investments into real estate.

How about you cash-out refinance the duplex, hire a management company, and then invest the money into an index fund?

Half of the ZIP codes across the Philadelphia market area recorded an Index in the High category in June while one-third were in the Moderate or Steady category.

Local banks and credit unions will usually base commercial lending off an index.

Garrett Gunderson's/Wealth Factory has what's called the Cash Flow Index, it's helped us determine a credit card and loan payoff plan, and may be useful to help you determine your opportunity cost when it comes to paying down low interest debt or pursue an investment that may return a higher rate. 

If you're interested in real estate investing, I would start with these:-Save your money by investing in indexes that mirror the S&P and NASDAQ.

I also agree that individual stocks can be risky, but indexed funds/ETFs I don't think are that risky.

The problem is I am 18, with about $10,000 in assets (only 6k I can use because I stored the rest in Roth IRA index funds) and have absolutely no idea where to start.

You can take a little of the profit to put into an index fund and pay taxes only on that part.