Deduplicate during _flatten_markers()

The potential downside of the approach in this series of commits is that
cnf() and dnf() can, in the worst case, be exponentially expensive.

eg `cnf((a1 and b1) or (a2 and b2) or ... (an and bn))` generates 2^n
intersections.

In practice, markers on python packages seem not to get long enough for
this to be an issue.

Deduplicating the markers on unions and intersections is the lowest of
low-hanging fruit in mitigating this.

src/poetry/core/version/markers.py

@@ -370,11 +370,14 @@ def _flatten_markers(
 
  for marker in markers:
  if isinstance(marker, flatten_class):
- flattened += _flatten_markers(
+ for marker in _flatten_markers(
  marker.markers, # type: ignore[attr-defined]
  flatten_class,
- )
- else:
+ ):
+ if marker not in flattened:
+ flattened.append(marker)
+
+ elif marker not in flattened:
  flattened.append(marker)
 
  return flattened