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
while (i.hasNext() && !buckets.isEmpty()) {
			MTrimContribution contribution = i.next();
			TrimKey key = getKey(contribution);
			ArrayList<MTrimContribution> slot = buckets.remove(key);
			if (slot == null) {
				continue;
			}
			MTrimContribution toContribute = null;
			for (MTrimContribution item : slot) {
				if (toContribute == null) {
					toContribute = item;
					continue;
				}
				Object[] array = item.getChildren().toArray();
				for (int c = 0; c < array.length; c++) {
					MTrimElement me = (MTrimElement) array[c];
					if (!containsMatching(toContribute.getChildren(), me)) {
						toContribute.getChildren().add(me);
					}
				}
			}
			if (toContribute != null) {
				toContribute.setWidget(null);
				result.add(toContribute);
			}
		}
		trace("mergeContributions: final size: " + result.size(), null); 
	}
}