Jeffrey P. answered 10/02/15
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No. There are an infinite number of counter examples to this conjecture.
19^2 = 361
x^2 + y^2 = 361
For any combination of x and y (the length and width of a rectangle) as long as the sum of each squared are equal to 361 will give you a rectangle with a diagonal of 19in. Any 2 different combinations would give you 2 non-congruent rectangles.