Why are there so many aircraft designs which look generally profoundly similar? The F-86, mig-15, Dassault Ouragan
The b-1 and tu-160
Concorde, tu-144, 2707

The low wing and T tail with two engines at the back. (CRJ, dc9)

The 'standard' shape with the mid aircraft wings, and vertical and horizontal stabilizers at the back, used from the E170 to the a380.

And while many general aviation craft are low winged or differ from a high wing tricycle single engine design, most seem to stick with that formula as well.

For some of the odder shapes, the question is 'industrial espionage?' but in the greater context, is it simple convergent evolution in that there are only so many shapes which are functional and useful so most things take on that general shape?
It’s an interesting question.  Aviation historian Bill Sweetman wrote about just that in an Air &amp; Space article called How the 747 Got Its Hump.  The answer to the question is quite long and specific to the aircraft type and use but the short and sweet answer is that airplanes are a great compromise between a number of factors, including mission profile, aerodynamics, structures, propulsion, avionics, manufacturability, realities and practicalities of product use, and on a few occasions customer insistence and industrial espionage.  See illustration below.



It just works out that, when all the factors above are taken into account, the designs you describe tend to be the best all around solutions for particular aircraft sizes, costs, payloads, mission profiles, safety, etc.  Like any other artifact made by humans, aircraft are but single answers to open ended design problems; a compromise between all of these competing requirements into a tangible product.  No one answer is 'correct' per se, but these kinds of aircraft designs seem to be the most optimal and every single design team when confronted with the requirements and design constraints create much the same thing.