2 identical fights at same time.

Say my flight in a CRK is full. Instead of upgrading to a 321/737 which I cant buy for another week, I may add a second exact flight at the same time on another CRK I already have. What are the disadvantages to this other than taking up another slot?

As you said you loose one valueable slot, and you loose margin/money. The 321 will be more profitable

Yep. Using one 321 means you only pay for 8 pilots.

Using two CRK means you have to pay for 16 pilots.

Yep. Using one 321 means you only pay for 8 pilots.

Using two CRK means you have to pay for 16 pilots.

Not to forget extra fleet management, and flight attendants. 

depending on your airline size and qty. of flights you offer on that route,

maybe you should consider to plan the second CRK in a scond time window

3 or 4 hour later. like that you will be able to have more transfer passengers

on other connections.

in that case it can make sense to fly with two aircraft instead of one.

While you save cost, you might not get a single passenger more with a bigger aircraft. The current demand system rewards frequency over size. The higher the frequency you have on a route, the more passengers you will grab and you’ll be less vulnerable to competitors grabbing your pax.

If you have one or two flights a day, but the competitor has 40, you’ll end up with empty planes.

So, yes, you save cost, but your overall profit might still be bigger with smaller aircraft but higher frequency.

Question:

If you do this at an airport with hardly any slots left, shouldn‘t that be considered slot blocking as well?

Maximizing your profit is the main objective of this game, no? Or is it to conserve scarce resources?

As long as there are plenty of slots left I'd go with the 2 CRKs