Ref: https://en.wikipedia.org/wiki/Squaring_the_square
This is a planar version of the question at Cubing the cube - as 'perfectly' as possible.
Question: How does one cut a square into the least number of squares that are all mutually non-congruent - except that 2 of the smaller squares are allowed to be congruent? One would hope that the number of square pieces would less than 21 when a pair of duplicates are allowed (21 being the least number of squares in a perfect squaring of the square, as discovered by Duijvestijn).
Note: It seems one can replace "2" above with "3" or "2 pairs" to generate other questions (but not "4" because that will trivialize the problem).
A possibly related question: Which is the rectangle that can be perfectly squared (cut into mutually non-congruent squares) with the least number of square pieces? This must be known stuff; am not able to locate a reference.