Since your microphone is active with phantom power turned on, and your mic pre is turned up to 12 o'clock, the audio playback from your speakers is being picked up by the microphone. You are hearing both the computer playback and the monitor output of the mic input at the same time. This creates the doubling effect. When you lower the volume on the slider, you're reducing the amount of output going to the speaker thus lowering the amount heard by the microphone.
To avoid hearing your microphone input monitor while playing back your audio, turn the mix knob to playback only (turn the knob hard right).
Then adjusting it back to Inputs when you want to record. Turn down your Audiobox output to avoid feeding back audio into the mic while recording. Use your headphones to monitor while recording instead.
The mix knob is explained in detail in the owners manual for the AudioBox USB.
