Details: 8 GB of RAM is installed as two 4 GB modules, no slots free.
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*Apple officially supports a maximum of 8 GB of RAM, but third-parties have determined that this model actually is capable of using up to 16 GB of RAM with two 8 GB memory modules.
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RAM Type: PC3-12800 DDR3L Min. RAM Speed: 1600 MHz
Sounds like this is your machine:
Apple MacBook Pro "Core i7" 2.9 13" Retina 2012 Specs
Identifiers: Retina Late 2012 - BTO/CTO - MacBookPro10,2 - A1425 - 2557
If so, there are no RAM slots and the machine is not upgradable.
http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.9-13-late-2012-retina-display-specs.html