Tax Sticker shock/ How to value a Duplex

[David Woz]

I have a Duplex in Fort Worth near the TCU area.  It is on the edge of the residential area that is "good" and the more sketchy part of Fort Worth.   The particular area has tons of Single Family, apartments and commercial, but very few duplex's and none that are remotely similar to mine.  (Mine is a 3/2 1800 Sq Ft. each side).

Just got my property tax and it went from 393K to 700K.   Yes, you read that right...   This would change it from positively flowing to me having to make a payment every month...

First, any advice on how to dispute, would be appreciated.  Has anyone actually paid for a professional appraisal?  

Second, Is there a "rule of thumb" on appraising a duplex in an area full of single family?   For example, worth 75% of two single family of same size/beds/baths?   

Just trying to wrap my head around this.

Finally, anyone want to buy a 700K Duplex!   LOL