Does 2.8GHZ Mid 2015 MacBook Pro come with dual graphics as default?
To answer your question "Does the 2.8GHz model come with dual Intel/AMD graphics card or just Intel embedded graphics only?", the 2.8GHz model came both ways.

The MacBook Pro (Retina, 15-inch, Mid 2015) with a 2.8GHz CPU was available with either an Intel Iris Pro Graphics​ only, or with the Intel Iris Pro Graphics​ and AMD Radeon R9 M370X Graphics, so you can't go by just the CPU.

The MacBook Pro (Retina, 15-inch, Mid 2015) with a 2.8GHz CPU and only the Intel Iris Pro Graphics​ has the Model Identifier: MacBookPro11,4

The MacBook Pro (Retina, 15-inch, Mid 2015) with a 2.8GHz CPU and both the Intel Iris Pro Graphics and AMD Radeon R9 M370X Graphics has the Model Identifier: MacBookPro11,5

So unless the Tech Specs provided by the seller states the Model Identifier as MacBookPro11,5 or states it has both the Intel Iris Pro Graphics and AMD Radeon R9 M370X Graphics then you'll have to ascertain additional information from the seller to determine what Graphics Adapters it has.

See here and here as references, as well as the official Tech Specs.

I also used information from Mactracker to determine and verify my statements.