seq: fix duplicate indices in sample_floyd

[PrarthanaPurohit]

Fix duplicate indices in sample_floyd

Problem

sample_floyd implements Floyd's combination algorithm to sample
amount distinct indices from 0..length. The algorithm has two cases
for each step:

• If the randomly picked value t is already in the set → add j instead
• If t is new → add t

The previous code unconditionally called indices.push(t) after the
if let block, so when t was a duplicate it was both replaced by j
at its old position and re-appended — producing duplicate indices
in the output.

Fix

Move push(t) into the else branch so it only runs when t is
a genuinely new element:

// Before
if let Some(pos) = indices.iter().position(|&x| x == t) {
    indices[pos] = j;
}
indices.push(t); // always ran — bug!

// After
if let Some(pos) = indices.iter().position(|&x| x == t) {
    indices[pos] = j;
} else {
    indices.push(t); // only when t is new
}