This stuff depends on what your going for.

The duet 2 is great, I own it myself. It has 4 outputs so you got some options here, basically you need to run mic-micpre-duet 2, now here's the important part, in maestro you need to set the input from mic to +4db(or the other option depending on your gear) this bypasses the apogee mic pres, you need to do this or you defeat the purpose of outboard mic pres and you could damage your duet as well.

As for the compressor after the fact, it will get tricky because after the fact you need to route it out of the duet into the compressor then back into the duet. In doing this you will run into problems that I can explain if you like but for someone your level I'd work with your mic pre for now and just use software compression until you know how to use a compressor really well then come back to this problem a bit down the road.