ParseLabelSelector round-trip instability with duplicate NotEquals values

[raballew]

Summary

ParseLabelSelector produces a NotIn expression with duplicate values when the same key appears in multiple != requirements. This breaks round-trip stability because the Kubernetes label parser uses a set internally, which silently drops duplicates.

Steps to reproduce

1. Parse "key!=,key!=" with ParseLabelSelector
2. Format the result with metav1.FormatLabelSelector -- produces "key notin (,)" (two empty strings joined by comma)
3. Parse "key notin (,)" again -- the Kubernetes parser stores values in a set, so the two empty strings collapse to one
4. Format again -- produces "key notin ()" instead of "key notin (,)"

Root cause

lease_helpers.go:129 blindly appends to the notEqualsByKey slice without deduplicating:

notEqualsByKey[key] = append(notEqualsByKey[key], values[0])

When the same key-value pair appears twice (e.g. key!=,key!=), the resulting Values slice contains ["", ""]. The Kubernetes labels.Parse function uses sets.String (a map) for values, which can only hold one copy of each value. So the first format is faithful to the slice, but the second parse deduplicates, and the second format reflects that.

Suggested fix

Deduplicate values in notEqualsByKey before building the NotIn expression, so the slice never contains duplicates that the set-based re-parser would collapse.

Found by

The FuzzParseLabelSelector fuzz test in PR #720.