Battery Charger Drawing Too Much Current
The Morningstar SunSaver Duo is designed for use with solar panels only, and is not specified for operation with an AC/DC power supply. It is a PWM solar charge controller. 

A PWM charge controller regulates battery voltage by switching the connection between the solar panel and battery on and off with a variable duty cycle.  The SunSaver Duo applies PWM at 10Hz or 300Hz, which is too low to effectively smooth out current pulses with an inductor. Peak current must therefore be limited by the solar panel itself.

To use a power supply you need some way to limit the peak current. The simplest way to do this is just insert a resistor in series. You can calculate the required value using Ohm's Law:- 

$$Resistance = \frac{Voltage}{Current}$$

For example if your PSU puts out 20V then \$\dfrac{20V-12V}{5A} = 1.6\Omega\$.

$$Power = Volts \times Amps$$

With the resistor dropping 8V at 5A it will dissipate \$8 \times 5 = 40W\$. You will need a large high power resistor, and it will get hot!