The problem does not define correct solution if same number is not in consecutive order, like x = [5 5 2 1 1 5 3]; Should it be three 5's, or two 5's then one 5? If it's an encoding problem the correct solution should be the last, right?

Typical DPCM encoding removes the DC and thus starts at 0, not 1. An 11% enhancement in encoding.

Good problem.

Most of the solutions with UNIQUE don't work when same number is not in consecutive order.

in the new test case 5, with x=[1 2 2 1], the test suite answer is [2 1 2 2] (two 1's, two 2's). I would expect the correct answer to be [1 1 2 2 1 1] (one 1, two 2's, one 1). I guess I am misinterpreting the question?

No. you are right. The test case 5 is wrong.

My mistake guys. Sorry about that. By the way, these comments are very helpful for adding (and fixing) bad test cases. Thanks for taking the time to comment.

Test case 5 answer is wrong. the correct answer is [2 1 2 2]. Change the test case 5 answer