Netclass: allow multiple assignments with the same pattern

This isn't really a problem, and overwriting the entry allows
for silent data loss by making a new assignment with the
same pattern as an existing one.

The resolve netclasses are de-duplicated later, so it's even
OK if the whole assignment is duplicated (in theory), but
continue to clean these up.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/21316
(cherry picked from commit db97d77544)

common/project/net_settings.cpp

@@ -558,15 +558,12 @@ bool NET_SETTINGS::HasNetclassLabelAssignment( const wxString& netName ) const
 
 void NET_SETTINGS::SetNetclassPatternAssignment( const wxString& pattern, const wxString& netclass )
 {
-    // Replace existing assignment if we have one
+    // Avoid exact duplicates - these shouldn't cause problems, due to later de-duplication
+    // but they are unnecessary.
     for( auto& assignment : m_netClassPatternAssignments )
     {
-        if( assignment.first->GetPattern() == pattern )
-        {
-            assignment.second = netclass;
-            ClearAllCaches();
+        if( assignment.first->GetPattern() == pattern && assignment.second == netclass )
             return;
-        }
     }
 
     // No assignment, add a new one