Do not duplicate stylesheets

Author: philippjfr

panel/theme/base.py

@@ -163,7 +163,8 @@ def _resolve_stylesheets(cls, value, defining_cls, inherited):
                 stylesheets.extend(inherited)
                 continue
             resolved = resolve_stylesheet(defining_cls, stylesheet, 'modifiers')
-            stylesheets.append(resolved)
+            if resolved not in stylesheets:
+                stylesheets.append(resolved)
         return stylesheets
 
     @classmethod