List of tuples without duplicates & repeated values

The accepted answer will quickly blow up with arguments of more than trivial sizes.

For example, with vals = {10, 20, 5, a, b, c} and n=10, it takes nearly two minutes to finish on my laptop, generating only 3003 results.

Better to generate the results directly, as a simple nested iteration:

f2[vals_, n_] := 
 With[{i = {#2, #1, Length@vals} & @@@ 
     Partition[Prepend[Array[i, n], 1], 2, 1]}, 
  Partition[vals[[Flatten[Table @@ {i[[All, 1]], Sequence @@ i}]]], n]];

This takes a few hundredths of a second to generate the same results from the aforementioned example.

It will also handle cases where the current answer will simply crash out with a "insufficient memory available" error (say 20 values and length 10).

n = 2;
vals = {0, 1};
Tuples[vals, {n}] // DeleteDuplicatesBy[#, Sort] &

As the comment said

Tuples[vals, {n}] // DeleteDuplicatesBy[Sort]

Also works, more clear.

Some explanations:

The key is: you "sort" the list to see whether they are duplicate.

So I use Sort to be DeleteDuplicatesBy's condition.