Bug 318713 Duplicate toolbar items (navigate group)
SDK 4.0 - I20100701-1105. 1. start fresh workspace ==> navigate group appears twice which is quite confusing and wastes space.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/ContributionsAnalyzer.java
return true;
		}
		return isVisible((MCoreExpression) contribution.getVisibleWhen(), eContext);
	}
	public static boolean isVisible(MTrimContribution contribution, ExpressionContext eContext) {
		if (contribution.getVisibleWhen() == null) {
			return true;
		}
		return isVisible((MCoreExpression) contribution.getVisibleWhen(), eContext);
	}
	static boolean isVisible(MCoreExpression exp, ExpressionContext eContext) {
		Expression ref = null;
		if (exp.getCoreExpression() instanceof Expression) {
			ref = (Expression) exp.getCoreExpression();
		} else {
			ref = new ReferenceExpression(exp.getCoreExpressionId());
			exp.setCoreExpression(ref);
		}
		try {
			return ref.evaluate(eContext) != EvaluationResult.FALSE;
		} catch (CoreException e) {
			trace("isVisible exception", e); 
		}
		return false;
	}
	public static void addMenuContributions(final MMenu menuModel,
			final ArrayList<MMenuContribution> toContribute,
			final ArrayList<MMenuElement> menuContributionsToRemove) {
		HashSet<String> existingMenuIds = new HashSet<String>();
		HashSet<String> existingSeparatorNames = new HashSet<String>();