Samsung Galaxy Duos

The Samsung Galaxy (stylized as Samsung GALAXY) is a series of Android-powered mobile computing devices designed, manufactured and marketed by Samsung Electronics, including Dual SIM SmartPhones marketed as "Duos".
Range.
The Samsung Galaxy offerings consist of a number of different ranges of devices, including SmartPhones, Phablets and Tablets. Although widely known by marketing names, the underlying model ranges are best understood by looking at the model number. Since August 2011, all the smartphones of the Galaxy series are categorized in the following way:
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where x = I, N or S and nnn = model number
Within the SmartPhone ranges, Samsung often produces a Dual SIM variant, but these on not often widely available in many markets where retail dominance by Mobile Network Operators (MNO) who operate a SIM-locked business model naturally deprecates Dual SIM devices.
The model number of Samsung SmartPhones will indicate the variant
Duos Models.
The following phones are available as Duos models
Samsung galaxy S8 Note also
Samsung "Dual SIM Always on" feature.
In their marketing materials Samsung use the term "Dual SIM Always on” to describe the Duos phones, although technically the term is misleading, since it does not mean quite what is says – both SIM cards are not always on. All phones with this feature are regular Dual SIM Stand-by (DSS) phones with 1 transceiver (radio) – 2nd SIM is always disconnected when a call is in progress on SIM 1 and vice versa.
The manual for such phones states: “Your device supports dual standby with two different networks. You cannot make or answer calls on both networks at the same time.”
These Android phones have a menu option “Dual SIM Always on” which when activated activates call forwarding on the carrier's network. This can be done manually on any phone, regardless of manufacturer, e.g. enable call forwarding to SIM 2 when SIM 1 is connected. Call Forwarding must be provided by the carrier, often for a fee, subscriber will also be charged for call forwarding on a minute by minute basis, this depends on the subscription agreement.
“Dual SIM Always on” might sound like it is enabling the dual SIM capability of the phone. However that control is elsewhere. Each SIM card in a Samsung dual SIM phone can be set to on or off independently in the top section of the SIM card Manager menu. When both are set to ON, then both are able to connect incoming calls, but only one at a time. If SIM 2 is in use, a caller to SIM 2 will be redirected by the carrier network as though the phone were off or out of range. E.g. to voicemail. This is the default behaviour, and it is what happens if “Dual SIM Always on” is set to "Offline ". 
Setting “Dual SIM Always on” allows a call on one SIM to interrupt a call on the other SIM. The menu option “Dual SIM Always on” might be better named “Dual SIM when busy”.