CMX-1 is the zoning the property is located in, but that zoning only dictates the different uses buildings are allowed to be by right. An existing duplex will fall under a by-right use in CMX-1 (in almost all circumstances, especially if bordered by other residential use).
The first thing I would do is go to http://property.phila.gov/ and look up the address and see its permit history (It may not have one. Don't be alarmed by that).
If it doesn't have a "use permit" as a duplex, you'll need to obtain one. Permits are granted by the office of Licenses and Inspections (L&I). You can obtain one by going to the lower concourse of the Municipal Services Building. You will have to fill out an application (here) and bring a check with you (the cost should be $125, but with Philly you never know).
Because the permit is by-right and is for an existing building, you won't need plans and you'll receive the permit the same day as you apply.
How do I know this? I work as an architect and expedite permits for my clients. I'm down at L&I at least twice a month dropping of dozens of plan sets. Let me know if you have any questions.